Terrorism and Immigration: A Risk Analysis

Including those murdered in the terrorist attacks of September 11, 2001 (9÷11), the chance of an American perishing in a terrorist attack on U.S. soil that was committed by a foreigner over the 41‐​year period studied here is 1 in 3.6 million per year. The hazard posed by foreigners who entered on different visa categories varies considerably. For instance, the chance of an American being murdered in a terrorist attack caused by a refugee is 1 in 3.64 billion per year while the chance of being murdered in an attack committed by an illegal immigrant is an astronomical 1 in 10.9 billion per year. By contrast, the chance of being murdered by a tourist on a B visa, the most common tourist visa, is 1 in 3.9 million per year. Any government response to terrorism must take account of the wide range of hazards posed by foreign‐​born terrorists who entered under various visa categories.

The federal government has an important role to play in screening foreigners who enter the United States, and to exclude those who pose a threat to the national security, safety, or health of Americans. This terrorism risk analysis of individual visa categories can aid in the efficient allocation of scarce government security resources to those categories that are most exploitable by terrorists. The hazards posed by foreign‐​born terrorists are not large enough to warrant extreme actions like a moratorium on all immigration or tourism.

Introduction

The December 2, 2015, terrorist attack that left 14 people dead in San Bernardino, California, was committed by American-born Syed Rizwan Farook and his foreign-born wife, Tashfeen Malik, who entered the United States two years earlier on a K-1 fiancé(e) visa.¹ Their attack was dramatic and brutal, and it prompted calls for heightened immigration restrictions, additional security checks for K-1 immigrants, and even a complete moratorium on all immigration.²

Substantial administrative hurdles and barriers are in place to block foreign-born terrorist infiltration from abroad.³ Any change in immigration policy for terrorism prevention should be subject to a cost-benefit calculation. A sensible terrorism screening policy must do more good than harm to justify its existence. That means the cost of the damage the policy prevents should at least equal the cost it imposes.

Government security resources should be allocated to the most efficient means of reducing the costs of terrorism. The Strategic National Risk Assessment (SNRA) seeks to evaluate the risk of threats and hazards, like terrorism, to help the government more effectively allocate security resources to the "threats that pose the greatest risk."⁴ However, the SNRA did not include a thorough terrorism risk analysis of different visa categories.

This policy analysis identifies 154 foreign-born terrorists in the United States who killed 3,024 people in attacks from 1975 through the end of 2015. Ten of them were illegal immigrants, 54 were lawful permanent residents (LPR), 19 were students, 1 entered on a K-1 fiancé(e) visa, 20 were refugees, 4 were asylum seekers, 34 were tourists on various visas, and 3 were from Visa Waiver Program (VWP) countries. The visas for 9 terrorists could not be determined. During that period, the chance of an American being murdered by a foreign-born terrorist was 1 in 3,609,709 a year. The chance of an American being killed in a terrorist attack committed by a refugee was 1 in 3.64 billion a year. The annual chance of being murdered by somebody other than a foreign-born terrorist was 252.9 times greater than the chance of dying in a terrorist attack committed by a foreign-born terrorist.

The first part of this policy analysis provides a quantification of the risks of foreign-born terrorists entering the United States in each U.S. visa category. It does so by identifying known foreign-born terrorists, counting how many people they murdered in terrorist attacks, and estimating the costs of those attacks. The second part of this policy analysis compares the costs of terrorism with the costs of proposed policy solutions such as an immigration moratorium.

Brief Literature Survey

Few researchers have tried to identify the specific visas used by terrorists, and none have used that information to produce a risk assessment for each U.S. visa category. John Mueller and Mark Stewart have produced superb terrorism risk analyses, but they did not focus specifically on the terrorism risk from visa categories.⁵ Robert S. Leiken and Steven Brooke wrote the most complete survey of visas used by foreign-born terrorists.⁶ However, their published work does not allow separating threats by country, their analysis ended in 2006, their data set is no longer available, and they did not produce a risk analysis.⁷

Broader links between immigration and terrorism are the subject of additional strands of research. Immigrants are overrepresented among those convicted of terrorist-related offenses post-9/11.⁸ In the developing world, heavy refugee flows are correlated with increased terrorism.⁹

Methodology

This analysis focuses on the 41-year period from January 1, 1975, to December 31, 2015, because it includes large waves of Cuban and Vietnamese refugees that posed a terrorism risk at the beginning of the time period and bookends with the San Bernardino terrorist attack. It identifies foreign-born terrorists who were convicted of planning or committing a terrorist attack on U.S. soil and links them with the specific visa they were first issued as well as the number of people they individually murdered, if any, in their attacks.¹⁰ This report counts terrorists who were discovered trying to enter the United States on a forged passport or visa as illegal immigrants. Asylum seekers usually arrive with a different visa with the intent of applying for asylum once they arrive, so they are counted under the asylum category. For instance, the Tsarnaev brothers, who carried out the Boston Marathon bombing on April 15, 2013, traveled here with a tourist visa but their family immediately applied for asylum, so they are included in that category.

Next, information on the individual terrorists, their visa types, and number of victims is compared with the estimated costs per victim and the total number of visas issued in each category. Where conflicting numerical estimates exist, the highest plausible figures are used with the intent to maximize the risks and costs of terrorism in terms of human life. The appendix lists all of the terrorists identified.

Finally, other costs of terrorism, such as property damage, losses to businesses, and reduced economic growth, are considered. Only three terrorist attacks committed by foreigners on U.S. soil have created significant property, business, and wider economic damage: the 1993 World Trade Center bombing, the 9/11 attacks, and the Boston Marathon bombing. The costs of the government's responses to terrorism are excluded. his analysis is concerned primarily with the cost of human lives taken in terrorist attacks.

Counting Foreign-Born Terrorists and Their Victims

This policy analysis examines foreign-born and immigrant terrorists and so excludes American-born terrorists except for purposes of comparison. For attacks planned or carried out by native-born Americans in concert with foreigners, the Americans are excluded and the immigrants are credited entirely for the terrorist plots and murders. That choice increases the estimates of the harm caused by foreign-born terrorists. For plots that included many foreign-born terrorists and victims, each terrorist is credited with an equal number of victims. For instance, the 1993 World Trade Center attack was committed by six foreign-born terrorists; six people were murdered, so each terrorist is responsible for one murder. Airplane hijackings that started in the United States and ended in different countries — such as the September 10, 1976, hijacking of TWA Flight 355 by Croatian nationalists that eventually terminated in Paris, France — are also included. However, this analysis excludes terrorist attacks in which the identities of the perpetrators were unknown, as well as attacks that occurred or were intended to occur (but were not successfully carried out) abroad.

Sources

The identities of the terrorists come from nine main data sets and documents. The first is Terrorism Since 9/11: The American Cases, edited by John Mueller.¹¹ This voluminous work contains biographical and other information related to attacks and cases since September 11, 2001. Mueller's work is indispensable because he focuses on actual terrorism cases rather than questionable instances of people who were investigated for terrorism but then cleared of terrorism, convicted under other statutes, and ultimately counted as "terrorism-related" convictions. For instance, the widely cited March 2010 Department of Justice (DOJ) report, National Security Division Statistics on Unsealed International Terrorism and Terrorism-Related Convictions,¹² included only 107 convictions based on actual terrorism statutes out of 399 "terrorism-related" convictions.¹³ Many of those terrorism-related convictions were for citizenship fraud, passport fraud, or false statements to an immigration officer by immigrants who never posed an actual terrorism threat to the homeland.¹⁴ The convictions of Nasser Abuali, Hussein Abuali, and Rabi Ahmed provide context for the government's use of the term "terrorism-related." An informant told the Federal Bureau of Investigation (FBI) that the trio tried to purchase a rocket-propelled grenade launcher, but the FBI found no evidence supporting the accusation. The three individuals were instead charged with receiving two truckloads of stolen cereal and convicted.¹⁵ The government classified their convictions as "terrorism-related" despite the lack of an actual terrorist connection, terror threat, planned attack, conspiracy, or any actual tentative steps taken toward carrying out a terror attack. That case is an especially absurd one to count as terrorism, but it is not too different from many of the other 289 convictions in the DOJ report.

The second source is the Fordham University Center on National Security's compilation of all of the trials for Islamic State of Iraq and Syria (ISIS) members in the United States.¹⁶ Third is the 2013 Congressional Research Service report American Jihadist Terrorism: Combating a Complex Threat.¹⁷ The fourth source of terrorist identities is the RAND Database of Worldwide Terrorism Incidents (RDWTI), which covers the years 1968-2009.¹⁸ Fifth is the Global Terrorism Database (GTD) maintained by the National Consortium for the Study of Terrorism and Responses to Terrorism at the University of Maryland, College Park.¹⁹ The RDWTI and GTD overlap considerably. Sources six through nine are the New America Foundation,²⁰Mother Jones,²¹ the Investigative Project on Terrorism,²² and the research of University of North Carolina professor Charles Kurzman.²³

Individual immigration information for the terrorists comes from the sources mentioned above, news stories, court documents, government reports, and publicly accessible databases. Many of the terrorists analyzed here entered the United States on one visa but committed their terrorist attack after they switched to another visa or were naturalized. This report classifies those individuals under the first visa they had when they entered. The only exception to that rule is for those seeking asylum in the United States — they are counted under the asylum visa. That exception is important because those individuals usually make their claim at the U.S. border or after they have entered on another visa, often with the intention of applying for asylum. For instance, Faisal Shahzad entered initially on a student visa and then obtained an H-1B visa before he unsuccessfully attempted to detonate a car bomb in Times Square in 2010. He is counted as having entered on a student visa.

The Attacks

These data sets identify 154 foreign-born terrorists in the United States from 1975 to the end of 2015. Ten of the subjects were illegal immigrants, 54 were lawful permanent residents (LPR), 19 were students, 1 entered on a K-1 fiancé(e) visa, 20 were refugees, 4 were asylum seekers, 34 were tourists on various visas, and 3 were from Visa Waiver Program (VWP) countries. The visas for 9 terrorists could not be determined.

The number of murder victims per terrorist attack comes from government reports, the RDWTI, the GTD, and John Mueller's research. From 1975 through 2015, those 154 foreign-born terrorists murdered 3,024 people, 98.6 percent of whom were killed on September 11, 2001. The other 1.4 percent of murder victims were dispersed over the 41- year period, with two spikes in 1993 and 2015. The spikes were produced by the 1993 World Trade Center bombing that killed 6 people and the combination of two 2015 incidents — the Chattanooga shooting on July 16, 2015, that killed 5 people and the San Bernardino attack on December 2, 2015, that killed 14 people. (The 2013 Boston Marathon bombing killed 3 people.)

From 1975 through 2015, the annual chance that an American would be murdered in a terrorist attack carried out by a foreign-born terrorist was 1 in 3,609,709. Foreigners on the Visa Waiver Program (VWP) killed zero Americans in terrorist attacks, whereas those on other tourist visas killed 1 in 3.9 million a year. The chance that an American would be killed in a terrorist attack committed by a refugee was 1 in 3.64 billion a year. Of the roughly 768,000 total murders committed in the United States from 1975 to the end of 2015, 3,024 (or 0.39 percent) were committed by foreign-born terrorists in an attack.²⁴ Those risk statistics are summarized in Table 1. The annual chance of being murdered was 252.9 times as great as dying in an attack committed by a foreign-born terrorist on U.S. soil.

The U.S. murder rate declined from a high of 10.17 per 100,000 in 1980 to a low of 4.45 per 100,000 in 2015 (see Figure 1). The 1975-2015 rate of murder committed by foreign-born terrorists was 0.026 per 100,000 per year, spiking to 1.047 in 2001. Zero Americans were killed in a domestic attack committed by foreign-born terrorists in 30 of the 41 examined years. In the 14 years after 9/11, only 3 years were marred by successful foreign-born terrorist attacks. Figure 1 shows a single perceptible blip for terrorism on the 9/11 attacks and a flat line otherwise.

Table 1. Chance of Dying in an Attack by a Foreign-Born Terrorist, 1975-2015

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; U.S. Census Bureau, "American Community Survey"; Disaster Center, "United States Crime Rates 1960-2014"; and author's calculations.

Note: Nonwhole numbers for deaths result from dividing the number of victims among multiple terrorist perpetrators.

Figure 1. U.S. Murder Rates, Excluding Foreign-Born Terrorism

Source: Disaster Center, "United States Crime Rates 1960-2014"; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; and author's calculations.

Uniqueness of 9/11

The foreign-born terrorist murder rate by itself has a single spike in 2001 and is virtually a flat line for every other year (see Figure 1). The foreign-born terrorist murder rate of 1.047 per 100,000 in 2001 is 176.3 times as great as the next highest annual rate of 0.0059 in 2015. The statistical mode (meaning the most common number) of the annual murder rate by foreign-born terrorists is zero.

The 9/11 attacks killed 2,983 people (not counting the 19 hijackers). The attacks were a horrendous crime, but they were also a dramatic outlier. The year 2015 was the deadliest year excluding 9/11, with 19 Americans killed by foreign-born terrorists. Fourteen of those victims were killed in the San Bernardino attack — the second deadliest ever committed by a foreign-born terrorist on U.S. soil. The attacks on 9/11 killed about 213 times as many people as were killed in San Bernardino.

To put the deaths by foreign-born terrorists into perspective, a total of 3,432 Americans were murdered in terrorist attacks during the 41-year time period. Of those, 408 were killed by native-born Americans or unknown terrorists, and 3,024 were killed by foreigners.²⁵

Government officials frequently remind the public that we live in a post-9/11 world where the risk of terrorism is so extraordinarily high that it justifies enormous security expenditures.²⁶ The period from 1975 to 2001 had only 17 murders committed by 16 foreign-born terrorists of a total of 64 who either tried or were successful in their attacks. During the same time period, 305 people were killed in terrorist attacks committed by native-born Americans and those with unknown nationalities. The majority of those victims (168) were killed in the 1995 Oklahoma City bombing that was committed by Timothy McVeigh and Terry Nichols, who were both U.S. natives.

From September 12, 2001, until December 31, 2015, 24 people were murdered on U.S. soil by a total of 5 foreign-born terrorists, while 65 other foreign-born terrorists attempted or committed attacks that did not result in fatalities. During the same period, 80 people were murdered in terrorist attacks committed by native-born Americans and those with unknown nationalities.

The number of murders committed by terrorists who are native-born or have unknown nationalities is higher than the number committed by foreigners in pre- and post-9/11 United States. The horrendous death toll from the terrorist attacks of 9/11 dominates deaths from other attacks.

Estimating the Cost per Terrorist Victim

When regulators propose a new rule or regulation to enhance safety, they are routinely required to estimate how much it will cost to save a single life under their proposal.²⁷ Human life is very valuable but not infinitely so. Americans are willing to take risks that increase their chance of violent death or murder, such as enlisting in the military, living in cities that have more crime than rural areas, or driving at high speeds, actions that would be unthinkable if individuals placed infinite value on their own lives. It then stands to reason that there is a value between zero and infinity that people place on their lives. In public policy, a review of 132 federal regulatory decisions concerning public exposure to carcinogens found that regulatory action never occurs if the individual fatality risk is lower than 1 in 700,000, indicating that risks are deemed acceptable if the annual fatality risk is lower than that figure.²⁸ A similar type of analysis for foreign-born terrorism will help guarantee that scarce resources are devoted to maximizing the number of lives saved relative to the costs incurred.

In 2010, the Department of Homeland Security (DHS) produced an initial estimate that valued each life saved from an act of terrorism at $6.5 million, then doubled that value (for unclear reasons) to $13 million per life saved.²⁹ Hahn, Lutter, and Viscusi use data from everyday risk-reduction choices made by the American public to estimate that the value of a statistical life is $15 million.³⁰ This policy analysis uses Hahn, Lutter, and Viscusi's $15 million estimate to remove any suspicion of undervaluation.

There are other costs of terrorism, such as property damage, medical care for the wounded, and disruptions of economic activity.³¹ However, those costs are highly variable and confined to three major terrorist attacks caused by foreigners. They are the 1993 World Trade Center bombing, the 9/11 attacks, and the Boston Marathon bombing. The highest plausible cost estimates for those events are $1 billion,³² $170 billion,³³ and $25 million,³⁴ respectively. The combined amount of just over $171 billion excludes the costs of the government's response to terrorism but captures virtually the entirety of the property and other economic damage. The cost of lives lost was greater than the value of property and other economic damages in every terrorist attack examined here except for the 1993 World Trade Center bombing and 9/11 attacks.

Terrorism Risk for Each Visa Category

The DHS annual Yearbook of Immigration Statistics³⁵ provided the statistics for the numbers of lawful permanent residents, student visas, K-1 fiancé(e) visas, asylum seekers, B-tourist visas, and entrants through the VWP. The numbers of student visas, K-1 fiancé(e) visas, and B-tourist visas issued are available from 1981 onward, and the VWP numbers are available only beginning in 1986, when the program was created. The particulars of the various visa programs will be described in their individual sections.

The Refugee Processing Center has recorded the number of refugees going back to 1975. The annual gross inflow of illegal immigrants is estimated on the basis of data from DHS, Pew Research Center, the Pew Hispanic Center, and other sources.³⁶ For the purposes of this report, only the illegal immigrants who actually entered the country illegally are included in that category.³⁷ Immigrants who entered on legal visas and became illegal by overstaying are counted under the legal visa category on which they entered. There are other vastly greater estimates of the number of illegal immigrants who entered the United States from 1975 to 2015; this analysis assumes the smaller estimated number of illegal entries to maximize the danger posed by that class of immigrants.³⁸ This estimation methodology could exaggerate the number of terrorists who entered the United States with an LPR status, thus diminishing the relative danger of other categories. At the time of writing, data were unavailable for 2014 or 2015, so for those years this paper uses an estimate of the previous two years of available visa numbers.

The terrorist risk for each visa category can be understood in different ways. The following sections will present the number of foreign-born terrorists in each visa category, the number of murders carried out by terrorists in each visa category, the chance of a terrorist getting a visa, and how many deaths can be expected by each foreign-born terrorist on a particular visa. Multiplying the number of murders in each visa category by the $15 million cost per victim yields the estimate of the costs of terrorism.

Each subsection that follows presents two estimates: one includes all victims from all foreign-born terrorist attacks from 1975 to the end of 2015 and the other excludes 9/11 because it is such an extreme outlier. The number of victims from the 9/11 attacks is more than two orders of magnitude greater than the next deadliest foreign-born terror attack on U.S. soil.³⁹ That scale of attack is unlikely to be repeated, whereas other attacks on a smaller and less deadly scale will certainly occur in the future. Presenting the terrorism hazard data in two formats, one including 9/11 and the other excluding it, enables the reader to focus on understanding the risks from the more common smaller-scale attacks that terrorists commit on U.S. soil.

Terrorism Risk for All Visa Categories

The U.S. government issued 1.14 billion visas under the categories exploited by 154 foreign-born terrorists who entered from 1975 to the end of 2015.⁴⁰ Of those, only 0.0000136 percent were actually granted to terrorists. In other words, one foreign-born terrorist entered the United States for every 7.38 million nonterrorist foreigners who did so in those visa categories. Table 2 and Figure 2 display these numbers, broken out in subcategories.

The 9/11 terrorist attacks were the deadliest in world history. Table 3 gives the same statistics as Table 2, except that it excludes the 9/11 attacks. Excluding the 9/11 terrorists and Zacarias Moussaoui, who intended to participate but could not because he was in jail at the time, 134 foreign-born terrorists entered the United States of a total of 1.14 billion visas issued in these categories from 1975 through 2015. That means that only 0.00001 percent of all foreigners who entered on these visas were terrorists. For each terrorist, excluding the 9/11 attackers, 8.48 million visas were granted to nonterrorist foreigners.

Of the 19 9/11 hijackers, 18 were on tourist visas. The 19th hijacker was Hani Hanjour who entered the United States on a student visa. Zacarias Moussaoui was not a hijacker on 9/11, but he was involved in the plot. His French citizenship allowed him to enter the United States on the VWP. Omitting the 9/11 terrorist attackers would make the student and the tourist visa categories look substantially safer and slightly improve the safety of the VWP.

Table 2. All Terrorists, by Visa Category, 1975-2015

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; Charles Kurzman, "Spreadsheet of Muslim-American Terrorism Cases from 9/11 through the End of 2015," University of North Carolina-Chapel Hill, http://kurzman.unc.edu/islamic-terrorism/; Department of Homeland Security; Pew Hispanic Research Center; Worldwide Refugee Admissions Processing System; and author's estimates.

Note: LPR = lawful permanent resident; VWP = Visa Waiver Program; K-1 = fiancé(e) visa; NA = Not available.
*1981 onward.
^1986 onward.

Figure 2. All Terrorists, by Visa Category

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; and Charles Kurzman, "Spreadsheet of Muslim-American Terrorism Cases from 9/11 through the End of 2015," University of North Carolina-Chapel Hill, http://kurman.unc.edu/islamic-terrorism/ .

Note: LPR = lawful permanent resident; VWP = Visa Waiver Program; K-1 = fiancé(e) visa.

Number and Cost of Terrorism Victims for All Visa Categories

As previously noted, 3,024 people were murdered by foreign-born terrorists in attacks in the United States from 1975 to the end of 2015. Those terrorist attacks cost $45.36 billion in human life or $1.11 billion per year on average as displayed in Table 4.⁴¹ The terrorism cost equals $39.93 per visa issued over that time.

Excluding the 9/11 terrorist attacks lowers the human cost of terrorism to $615 million during the period or $15 million per year as displayed in Table 5. The murder-cost of terrorism committed by the foreign-born inside the United States, excluding 9/11, is $0.54 per visa issued.

Of the 154 terrorists, 114 did not murder anyone in a terrorist attack. Many of them were arrested before they were able to execute their attacks or their attacks failed to take any lives. Including all terrorists and the 9/11 hijackers, even the ones who did not kill anybody, each terrorist killed about 20 people on average for a total human cost of $294.6 million. Excluding 9/11, each terrorist killed an average of 0.31 people, for a total human cost of $4.6 million per terrorist analyzed here.

Only 40 of the 154 foreign-born terrorists actually killed anyone. Of those terrorists, each one killed an average of 75.6 people and took $1.13 billion worth of human life. Excluding 9/11, each successful terrorist killed an average of just under two people for a human cost of $29.29 million inflicted by each successful terrorist.

Excluding the 9/11 attackers, 21 foreign-born terrorists succeeded in murdering 41 people from 1975 through 2015. Sixteen of those terrorists committed their attacks prior to 9/11 and killed a total of 17 people — an average of 1.06 murders per terrorist. Only two terrorists during this time period killed more than one person each: Mir Aimal Kasi shot and killed Central Intelligence Agency (CIA) employees Frank Darling and Lansing Bennett as they were waiting in traffic outside of CIA headquarters in Langley, Virginia, on January 25, 1998; and El Sayyid Nosair assassinated Meir Kahane on November 5, 1990, and then participated in the first World Trade Center attack on February 26, 1993, which killed six people. Over time the number of terrorists has shrunk but their deadliness has increased.

Table 3. Terrorists, by Visa Category, Excluding 9/11 Attacks, 1975-2015

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; Charles Kurzman, "Spreadsheet of Muslim-American Terrorism Cases from 9/11 through the End of 2015," University of North Carolina-Chapel Hill, http://kurzman.unc.edu/islamicterrorism/; Department of Homeland Security; Pew Hispanic Research Center; Worldwide Refugee Admissions Processing System; and author's estimates.

Note: LPR = lawful permanent resident; VWP = Visa Waiver Program; K-1 = fiancé(e) visa; NA = Not available.
*1981 onward.
^1986 onward.

Table 4. Deadliness of All Terrorists, by Visa Category, 1975-2015

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; Department of Homeland Security; Pew Hispanic Research Center; Worldwide Refugee Admissions Processing System; and author's estimates.

Note: Nonwhole numbers for deaths result from dividing the number of victims among multiple terrorist perpetrators; NA = Not available.

Table 5. Deadliness of All Terrorists, by Visa Category, Excluding 9/11, 1975-2015

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; Department of Homeland Security; Pew Hispanic Research Center; Worldwide Refugee Admissions Processing System; and author's estimates.

Note: Nonwhole numbers for deaths result from dividing the number of victims among multiple terrorist perpetrators; NA = Not available.

There were five successful attacks after 9/11 that killed 24 people, with each terrorist responsible for an average of 4.8 murders. Egyptian-born Hesham Mohamed Hedayet killed two people on July 4, 2002, at Los Angeles International Airport; the Tsarnaev brothers killed three people in the Boston Marathon bombing on April 15, 2013; Mohammad Abdulazeez murdered five people on July 16, 2015; and Tashfeen Malik, along with her U.S.-born husband, killed 14 on December 2, 2015, in San Bernardino, California. The pre-9/11, 9/11, and post-9/11 numbers are summarized in Table 6.

Table 6. Foreign-Born Terrorists and Murders in Pre- and Post-9/11 United States

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; Department of Homeland Security; Pew Hispanic Research Center; Worldwide Refugee Admissions Processing System; and author's estimates.

Foreign-born terrorists on tourist visas have killed more Americans in attacks than those on any other type of visa, followed distantly by those who entered on student visas. The 2,983 deaths on 9/11 account for all but 41 of those deaths. Excluding the 9/11 attacks, the K-1 fiancé(e) visa appears to be the deadliest (due entirely to the San Bernardino attack) followed by LPRs and tourists.

The following subsections discuss the terrorism risks and costs for each specific visa category. Summary data for the categories are provided in Table 7.

Table 7. Summary of Terrorism Incidents and Costs, by Visa Category

Sources: John Mueller, ed., Terrorism Since 9/11: The American Cases; RAND Database of Worldwide Terrorism Incidents; National Consortium for the Study of Terrorism and Responses to Terrorism Global Terrorism Database; Center on National Security; Charles Kurzman, "Spreadsheet of Muslim-American Terrorism Cases from 9/11 through the End of 2015," University of North Carolina-Chapel Hill, http://kurzman.unc.edu/islamic-terrorism/; Department of Homeland Security;Pew Hispanic Research Center; Worldwide Refugee Admissions Processing System; and author's estimates.

Illegal Immigrants

Only 10 illegal immigrants became terrorists, a minuscule 0.000038 percent of the 26.5 million who entered from 1975 through 2015 as summarized in Table 7. In other words, 2.65 million illegal immigrants entered the United States for each one who ended up being a terrorist.

Only one of those illegal immigrants, Ahmed Ajaj, actually succeeded in killing an American as he was one of the 1993 World Trade Center conspirators. The human cost of terrorism caused by illegal immigrants was thus $15,000,000 or equal to $0.57 cents per illegal immigrant. As a reminder, none of the 9/11 hijackers entered the United States illegally.

Lawful Permanent Residents

An LPR is also commonly known as a green card holder. An LPR can reside and work permanently in the United States until such time as he naturalizes or commits a serious enough crime to lose his green card and be deported.⁴²

More terrorists have taken advantage of the LPR category than any of the other visa categories. From 1975 through 2015, 54 foreign-born terrorists were LPRs — an average of 1.32 terrorists per year. Over the 41-year period, more than 35 million LPRs were allowed in, meaning that just 0.00016 percent of LPRs were actual terrorists. In other words, one terrorist entered for every 644,990 nonterrorist legal permanent residents.

Those 54 LPR terrorists killed only eight people in terrorist attacks. The human cost of LPR terrorism was thus $120 million, equal to $3.45 per green card issued. None of the 9/11 hijackers had green cards.

Student Visas

Student visas allow foreigners to enter the United States temporarily to attend an educational institution such as a college, university, seminary, private elementary school, or vocational training program.⁴³

A total of 19 students — 0.00008 percent of the 24,176,617 student visas issued from 1981 to 2015 — were terrorists.⁴⁴ In other words, one terrorist was issued a student visa for every 1,272,454 students who were not terrorists.

Terrorists on student visas appear especially deadly because one of them was a 9/11 hijacker. Altogether, students caused 158.5 fatalities, or one for every 152,534 students admitted.⁴⁵ The human cost of terrorism caused by foreigners on student visas was thus $2.38 billion, equal to 5.23 percent of all the terrorism costs to human life. The average terrorism cost per student visa issued is $98.34.

Excluding 9/11, 18 terrorists entered the United States as students, or one entry for every 1.34 million student visas issued. Those 18 committed a total of 1.5 murders that cost $22.5 million or $0.93 per student visa issued.

K-1 Fiancé(e) Visas

The K-1 visa permits a foreign-citizen fiancé or fiancée to travel to the United States to marry his or her U.S.-citizen sponsor within 90 days of arrival. Once married, the foreign-citizen can then apply to adjust his or her immigration status to that of LPR.⁴⁶

Tashfeen Malik entered the United States on a K-1 visa sponsored by her U.S.-born husband, Syed Rizwan Farook. Together they murdered 14 people during the San Bernardino terrorist attack of December 2, 2015. Because it is unknown which attacker specifically killed which victims, this report attributes all 14 murders to Malik.

The San Bernardino attack is the only one to involve this visa. However, because of the relatively small number — 604,132 — of these visas issued over the 41-year time frame examined, this lone attack makes the K-1 look like a very dangerous visa, with a single murder for every 43,152 K-1 visas issued. The single terrorist on the K-1 visa has imposed $210 million in costs or an average of $347.61 for every K-1 visa issued — by far the highest cost per visa issued. So while it is the second deadliest visa, there is no trend of K-1 visa holders committing attacks.⁴⁷

Refugees

A refugee is a person who is located outside of the United States and is of special humanitarian concern; demonstrates that he or she was persecuted or fears persecution because of race, religion, nationality, political opinion, or membership in a particular social group; is not firmly settled in another country; and does not violate other immigration bars on admission such as posing a national security or public health risk.⁴⁸ Refugees apply from a third country and then enter the United States after they have been granted their visa. Refugees must apply for a green card after one year of residing in the United States.

Of the 3,252,493 refugees admitted from 1975 to the end of 2015, 20 were terrorists, which amounted to 0.00062 percent of the total. In other words, one terrorist entered as a refugee for every 162,625 refugees who were not terrorists. Refugees were not very successful at killing Americans in terrorist attacks. Of the 20, only three were successful in their attacks, killing a total of three people and imposing a total human cost of $45 million, or $13.84 per refugee visa issued. The three refugee terrorists were Cubans who committed their attacks in the 1970s and were admitted before the Refugee Act of 1980 created the modern rigorous refugee-screening procedures currently in place. Prior to that act, a hodgepodge of poorly managed post-World War II refugee and displaced persons statutes, presidential grants of parole, and ad hoc congressional legislation allowed Hungarian, Cuban, Vietnamese, and other refugee groups to settle in America.⁴⁹ All of the murders committed by foreign-born refugees in terrorist attacks were committed by those admitted prior to the 1980 act.

Two of the Cuban terrorists assassinated a Chilean dissident and his American aide. The third Cuban terrorist assassinated a Cuban exile leader who supported a closer United States relationship with Fidel Castro. The GTD and RDWTI showed many more terrorist attacks and assassinations in the 1970s and 1980s that were likely perpetrated by Cuban or Vietnamese refugees, but no one was ever arrested for the crimes so they could not be included here.

Many of the refugees arrested after 9/11 were admitted as children, and in some cases there is doubt over whether their attacks even qualify as terrorism.⁵⁰ Other refugees have been arrested for terrorism or the vague "terrorism-related charges," but they were planning terrorist attacks overseas or providing material support for foreign groups operating overseas.⁵¹ No refugees were involved in the 9/11 attacks.

Asylum Seekers

Asylum seekers are those who ask U.S. border officials for protection because they have suffered persecution or fear that they will suffer persecution because of their race, religion, nationality, membership in a particular social group, or political opinions.⁵² Unlike refugees, asylum seekers must apply in person at the border and are often detained before being granted asylum. Four asylum seekers, or 0.0006 percent of the 700,522 admitted from 1975 through 2015, later turned out to be terrorists. For every terrorist who was granted asylum, 175,131 nonterrorist asylum seekers were admitted.

Terrorists who were asylum seekers killed four people in terrorist attacks, three of them in the Boston Marathon bombing on April 15, 2013, carried out by the Tsarnaev brothers. The brothers entered the United States as young children and later became terrorists. Ramzi Yousef, who helped plan the 1993 World Trade Center bombing that killed six people, was the other asylum seeker. Because Yousef planned and carried out those attacks as a member of a six-person team, this report considers him to be responsible for one of the six murders.

Altogether, asylum seekers caused four fatalities, or one for every 175,131 admitted. The total human cost of terrorism by asylum seekers was $60 million, equal to an average of $85.65 per asylum seeker admission. No asylum seekers were involved with 9/11.

Tourist Visas

Tourists on the B visa are allowed to tour the United States for business or pleasure as well as enroll in short recreational courses of study.⁵³ These are the tourist visas available to most residents of the world.

The tourist visa categories were the second most abused by terrorists. A total of 34 terrorists entered the United States on tourist visas over the 35-year period (1981-2015) for which data are available. That is an average of 0.97 terrorists who entered on a tourist visa annually. Almost 658 million tourists entered the United States on tourist visas, so a single terrorist was issued a visa in this category for every 19.35 million issued.

The 34 terrorists on tourist visas killed 2,834 people in attacks or one victim for every 232,157 visas issued. The total terrorism cost in terms of human life by terrorists on tourist visas was $42.51 billion, or $64.61 per visa.

Eighteen of the terrorists who carried out the 9/11 attacks held tourist visas, so this visa category is responsible for 93.7 percent of all deaths caused by terrorists. Excluding 9/11 lowers the number of fatalities to eight and the total death-related costs to $120 million or $0.18 per tourist visa issued. Excluding the 9/11 hijackers, one terrorist entered on a tourist visa for every 41.12 million nonterrorist tourists. There was one murder victim for every 82.24 million nonterrorist tourists who entered.

Visa Waiver Program

The VWP enables most citizens of the participating countries to travel to the United States for business or tourism for up to 90 days without first obtaining a visa.⁵⁴ The participating countries are developed nations in Europe, East Asia, and South America that have established security procedures to exclude terrorists and share traveler information with the U.S. government, and whose citizens rarely overstay illegally in the United States.⁵⁵

There were three terrorists on the VWP out of a total of 388 million entries during the life of the program (since 1986), or a single terrorist for every 129 million entries. That makes the VWP the safest visa category. The three VWP terrorists killed zero people. One was French national Zacarias Moussaoui, who was originally part of the 9/11 conspiracy but was in jail on unrelated charges during the attacks. The second was the British shoe bomber Richard Reid, who attempted to ignite his shoe on a transatlantic flight en route to the United States. The last was Qaisar Shaffi, who cased New York buildings for a future attack that was broken up by British intelligence. Besides those three, Ahmed Ajaj and Ahmed Ressam were apprehended at John F. Kennedy International Airport while attempting to enter the country illegally using forged passports from nations that were part of the VWP. Because they were captured at the border and their documents were forgeries, they are classified as illegal immigrants.⁵⁶

In addition, a few international terrorist suspects have been apprehended while trying to enter through the VWP. These include a member of the Provisional Irish Republican Army, a French-Bolivian dual-national who was implicated in a 1990 bombing of U.S. Marines in La Paz, and a British mercenary who tried to buy a fighter jet for the infamous Colombian drug lord Pablo Escobar.⁵⁷

According to the historical data, the VWP was the least likely category to be used by terrorists.

Unknown

The visa statuses of nine terrorists are unknown. Those individuals committed their attacks or were arrested between 1975 and 1990. Only two of the nine actually succeeded, and they are responsible for 1.5 murders with a total human cost of $22.5 million.⁵⁸

Cost-Benefit Analysis

Immigration screening for counter-terrorism purposes is important, but it will never be perfect.⁵⁹ As Steven Camarota at the Center for Immigration Studies wrote, "To be sure, in a nation as large as the United States, it is impossible to prevent terrorists from entering the country 100 percent of the time."⁶⁰ Even though terrorists rarely achieve their ultimate policy goals, the United States will always be vulnerable to terrorist attacks in the sense that the possibility of harm will be greater than zero.⁶¹

Confronted with the threat of Islamic terrorism, well-known conservatives like Larry Kudlow, David Bossie, and Ann Coulter have called for a complete moratorium on immigration.⁶² They presumably want to restrict only LPRs, student visas, fiancé(e) visas, illegal immigrants, refugees, and asylum seekers, but they may also want to prevent the entry of tourists. The following sections will separate tourists from immigrants and migrants to estimate how many Americans must die from terrorism to justify a moratorium on foreigners entering the United States. Finding the break-even point at which the benefits of reduced terrorism justify the cost incurred by stopping all legal immigration and tourism helps form the outer-most boundaries of a sensible policy.⁶³ If the benefits of the different policies proposed below outweigh the costs, then the measure is cost-effective. If, however, the costs of the policies proposed below are greater than the benefits, then they are not cost-effective.

This cost-benefit analysis considers the cost of human deaths, property damage, injuries, and economic disruption caused by terrorism. In virtually all cases of terrorism, with the notable exception of the 9/11 attacks, property damage is minuscule while the cost of injuries is minor compared with the cost of the deaths. Government reactions to terrorism, such as the virtual shutdown of Boston in the wake of the Marathon bombing and the grounding of all air travel after 9/11, are not considered.⁶⁴

Broad Immigration Moratorium

The economic cost of a moratorium on all future immigration is tremendous. Professor Benjamin Powell of Texas Tech University estimated the economic costs of a total immigration moratorium at $229 billion annually.⁶⁵

This section includes two cost projections. The first conservatively estimates the economic costs of a moratorium to be only $35 billion annually, which is the number used by Harvard economist George Borjas.⁶⁶ That $35 billion counts only the immigration surplus, which is the increase in American wages caused by immigration. The figure ignores other enormous economic benefits, including the economic gains to the immigrants themselves. The second cost projection assumes the $229 billion annual price tag of a moratorium calculated by Benjamin Powell.

The greatest possible benefit of an immigration moratorium would be the elimination of all terrorism by immigrants. Including the devastation caused by the 9/11 attacks, 190 people were murdered on U.S. soil in terrorist attacks committed by 117 illegal immigrants, LPRs, students, fiancé(e)s, refugees, asylum seekers, and those with unknown visa statuses since 1975 — accounting for 6.3 percent of all fatalities caused by foreign-born terrorists on American soil. The other 2,834 murders, or 93.7 percent, were committed by 34 tourists who would have been unaffected by an immigration moratorium. Those 34 tourists account for 22.1 percent of all foreign-born terrorists but 93.7 percent of murders caused by foreign-born terrorist attacks. Some 99.7 percent of the murders committed by terrorists on tourist visas occurred on 9/11. A ban on immigration will barely diminish the costs of terrorism.

The costs of an immigration moratorium vastly exceed the benefits, even with very generous assumptions buttressing the pro-moratorium position. According to a break-even analysis, which seeks to find when the cost of an immigration restriction would equal the benefit of reduced terrorism, an immigration moratorium would have to prevent 2,333 deaths annually at an estimated $15 million per death. In reality, an average of 4.6 murders were committed per year by immigrant (non-tourist) terrorists during the 41-year period. An immigration moratorium would have to prevent 504 times as many such murders in any given year as actually occurred annually from 1975 through 2015 for the costs of a moratorium to equal the benefits.

Benjamin Powell's more realistic $229 billion annual estimate of the economic costs of an immigration moratorium means the ban would have to prevent 15,267 murders by terrorists each year at a cost savings of $15 million per murder for the benefits of the ban to equal the costs. That number is about 3,294 times as great as the average annual number of terrorist deaths caused by immigrants (excluding tourists) and more than five times as great as all of the murders committed by all foreign-born terrorists (including tourists) from 1975 through 2015.

In short, an immigration moratorium produces huge economic costs for minuscule benefits.

Tourism Moratorium

Given the role that tourism played in the 9/11 attacks, it is tempting to think that limiting an immigration ban to tourism might be a preferable policy. Yet the economic costs of a tourism moratorium are even larger. The World Travel and Tourism Council estimated that international tourists added $194.1 billion directly and indirectly to the U.S. economy in 2014.⁶⁷ A moratorium on tourism would deny the U.S. economy an amount of economic activity equal to just over 1 percent of U.S. gross domestic product.

The majority of all murders committed by foreign-born terrorists, 93.7 percent, were committed by 34 different terrorists on tourist visas. A total of 99.7 percent of all terrorist murders committed by those on tourist visas were committed by 18 such men on 9/11. Over the entire 41-year period of this study, an average of 69.1 Americans were murdered each year in terrorist attacks committed by those on tourist visas, producing an average annual cost of $1.037 billion — which is what would be saved if there was a moratorium.

But the costs of a tourist moratorium vastly exceed the benefits. Such a moratorium would have to deter at least 12,940 murders by terrorists per year to justify the loss in economic activity. The annual number of murders committed by tourists in terrorist attacks would have to be 187.2 times as great as they currently are to justify a moratorium. To put in perspective the 12,940 murders that would have to be prevented each year, that is about 4.3 times as great as all the deaths caused by all foreign-born terrorists over the entire 41-year period studied here. Counterterrorism cannot justify a tourist moratorium.

Including Nonhuman Costs

The destruction of private property, businesses, and economic activity caused by foreign-born terrorism during the 1975-2015 time period is estimated to have cost $171 billion. The combined human, property, business, and economic costs of terrorism from 1975 through 2015 are thus estimated at $216.39 billion. Spread over 41 years, the average annual cost of terrorism is $5.28 billion, which is still far less than the minimum estimated yearly benefit of $229.1 billion from immigration and tourism ($35 billion + $194.1 billion). The average yearly costs of terrorism, including the loss of human life, injuries, property destruction, and economic disruptions, would have to be 43.4 times as great as they have been to justify a moratorium on all foreigners entering the United States. A moratorium on foreigners entering the United States is more costly than the benefits even when including the property, business, and greater economic costs caused by foreign-born terrorism.

Conclusion

Foreign-born terrorism on U.S. soil is a low-probability event that imposes high costs on its victims despite relatively small risks and low costs on Americans as a whole.⁶⁸ From 1975 through 2015, the average chance of dying in an attack by a foreign-born terrorist on U.S. soil was 1 in 3,609,709 a year. For 30 of those 41 years, no Americans were killed on U.S. soil in terrorist attacks caused by foreigners or immigrants. Foreign-born terrorism is a hazard to American life, liberty, and private property, but it is manageable given the huge economic benefits of immigration and the small costs of terrorism. The United States government should continue to devote resources to screening immigrants and foreigners for terrorism or other threats, but large policy changes like an immigration or tourist moratorium would impose far greater costs than benefits.

A. Appendix

All identified foreign persons who attempted terrorism in the United States over the time period 1975-2015 are listed in Table A1.

Table A.1. Identified Foreign Persons Who Attempted or Committed Terrorism on U.S. Soil, 1975-2015

Note: A=asylee, F=student on F or M visa, I=illegal, K=K-1 fiancé(e), L=lawful permanent resident, R=refugee, T=tourist on B-visa,

U=unknown, V=visa waiver program.

*If multiple attackers, all casualties spread evenly across all attackers.

Notes

1. Matt Pearce, "A Look at the K-1 Visa That Gave San Bernardino Shooter Entry into U.S.," Los Angeles Times, December 8, 2015, http://www.latimes.com/world/europe/la-fg-k1-visas-20151208-story.html.

2. Alicia A. Caldwell, "U.S. Reviewing Fiancé Visa Program after San Bernardino Shooting," Associated Press, December 8, 2015, http://www.pbs.org/newshour/rundown/u-s-reviewing-fiance-visa-program-after-san-bernardino-shooting/; Larry Kudlow, "I've Changed. This Is War. Seal the Borders. Stop the Visas," National Review, December 11, 2015, http://www.nationalreview.com/article/428411/larry-kudlow-seal-borders-stop-visas; David Bossie, "Conservatives Should Think Bigger on Immigration Ban," Breitbart, December 11, 2015, http://www.breitbart.com/big-government/2015/12/11/conservatives-should-think-bigger-on-immigration-ban/; Ann Coulter, interview by Breitbart News Saturday, December 12, 2015, https://soundcloud.com/breitbart/breitbart-news-saturday-ann-coulter-december-12-2015.

3. See Jared Hatch, "Requiring a Nexus to National Security: Immigration, ‘Terrorist Activities,' and Statutory Reform," BYU Law Review 3 (2014): 697-732.

4. U.S. Department of Homeland Security [DHS], "The Strategic National Risk Assessment in Support of PPD 8: A Comprehensive Risk-Based Approach toward a Secure and Resilient Nation" (Washington: DHS, December 8, 2011), https://www.dhs.gov/xlibrary/assets/rma-strategic-national-risk-assessment-ppd8.pdf.

5. John Mueller, ed., Terrorism Since 9/11: The American Cases (Columbus: Ohio State University, March 2016), http://politicalscience.osu.edu/faculty/jmueller/since.html.

6. Robert S. Leiken and Steven Brooke, "The Quantitative Analysis of Terrorism and Immigration: An Initial Exploration," Terrorism and Political Violence 18, no. 4 (2006): 503-21.

7. Emails with Robert Leiken on March 14, 2016, and Steven Brooke on March 17, 2016, confirmed that the data set their paper was based on does not exist anymore. Emails are available upon request.

8. U.S. Government Accountability Office [GAO], "Criminal Alien Statistics: Information on Incarcerations, Arrests, and Costs," GAO-11-187 (Washington: GAO, March 2011), p. 25.

9. Daniel Milton, Megan Spencer, and Michael Findley, "Radicalism of the Hopeless: Refugee Flows and Transnational Problems," International Interactions (August 2013): 3.

10. Illegal immigrants are included in a visa category called "illegal" to improve readability.

11. Mueller, Terrorism and Political Violence.

12. U.S. Department of Justice [DOJ], National Security Division Statistics on Unsealed International Terrorism and Terrorism-Related Convictions 9/11/01-3/18/10 (Washington: DOJ), https://fas.org/irp/agency/doj/doj032610-stats.pdf.

13. GAO, "Criminal Alien Statistics," pp. 25-26.

14. DOJ, National Security Division Statistics.

15. "Profiles in Terror: Nasser Abuali," Mother Jones, http://www.motherjones.com/fbi-terrorist/nasser-abuali-stolen-cereal.

16. "By the Numbers: ISIS Cases in the United States, March 1, 2014-January 25, 2016," New York, Center on National Security at Fordham Law, January 25, 2016, http://static1.squarespace.com/static/55dc76f7e4b013c872183fea/t/56a7a90a2399a387c5bc9eeb/1453828362342/ISIS+Cases-+Statistical+Overview+01-25-16.pdf.

17. Jerome P. Bjelopera, "American Jihadist Terrorism: Combating a Complex Threat," CRS Report for Congress no. R41416 (Washington: Congressional Research Service, January 23, 2013), https://www.fas.org/sgp/crs/terror/R41416.pdf.

18. RAND National Security Division, "RAND Database of Worldwide Terrorism Incidents," http://www.rand.org/nsrd/projects/terrorism-incidents.html.

19. "Global Terrorism Database" (College Park: University of Maryland), http://www.start.umd.edu/gtd/.

20. "Homegrown Extremism 2011-2015" (Washington: New America Foundation), http://securitydata.newamerica.net/extremists/analysis.html.

21. "Profiles in Terror," Mother Jones, http://www.motherjones.com/fbi-terrorist.

22. The Investigative Project on Terrorism, "International Terrorism and Terrorism-Related Convictions 9/11/01-3/18/10," http://www.investigativeproject.org/documents/misc/627.pdf.

23. Charles Kurzman, "Spreadsheet of Muslim-American Terrorism Cases from 9/11 through the End of 2015," University of North Carolina-Chapel Hill, http://kurzman.unc.edu/islamic-terrorism/.

24. United States Crime Rates 1960-2014, Disaster Center website, http://www.disastercenter.com/crime/uscrime.htm.

25. I used the Global Terrorism Database (GTD) at the University of Maryland to estimate the total number of terrorist deaths in the United States during this time period except for 1993 because the data are missing for that year. I used data from the RAND Database to fill in the missing 1993 GTD data.

26. John Mueller and Mark G. Stewart, Chasing Ghosts: The Policing of Terrorism (New York: Oxford University Press, 2016), pp. 13-21.

27. John Mueller and Mark G. Stewart, "Responsible Counterterrorism Policy," Cato Institute Policy Analysis no. 755, September 10, 2014, p. 4.

28. Mueller and Stewart, Chasing Ghosts, p. 137.

29. Lisa A. Robinson, James K. Hammitt, Joseph E. Aldy, Alan Krupnick, and Jennifer Baxter, "Valuing the Risk of Death from Terrorist Attacks," Journal of Homeland Security and Emergency Management 7, no. 1 (2010): article 14.

30. Robert W. Hahn, Randall W. Lutter, and W. Kip Viscusi, "Do Federal Regulations Reduce Mortality?" Washington, AEI-Brookings Joint Center for Regulatory Studies, 2000, https://law.vanderbilt.edu/files/archive/011_Do-Federal-Regulations-Reduce-Mortality.pdf. See also Benjamin H. Friedman, "Managing Fear: The Politics of Homeland Security," Political Science Quarterly 126, no. 1 (2011): 85, footnote 31.

31. See Karen C. Tumlin, "Suspect First: How Terrorism Policy Is Reshaping Immigration Policy," California Law Review 92, no. 4 (July 2004): 1173-239; and John Mueller and Mark G. Stewart, "Evaluating Counterterrorism Spending," Journal of Economic Perspectives 28, no. 3 (Summer 2014): 237-48.

32. Phil Hirschkorn, "New York Remembers 1993 WTC Victims," CNN New York Bureau, February 26, 2003, http://www.cnn.com/2003/US/Northeast/02/26/wtc.bombing/.

33. Mueller and Stewart, Chasing Ghosts, pp. 144, 279.

34. "Insurers Have Paid $1.2M for Boston Bombing P/C Claims So Far; Health Claims to Top $22M," Insurance Journal, August 30, 2013, http://www.insurancejournal.com/news/east/2013/08/30/303392.htm.

35. U.S. Department of Homeland Security, "Yearbook of Immigration Statistics" (Washington: DHS, multiple years), https://www.dhs.gov/yearbook-immigration-statistics.

36. Thomas J. Espenshade, "Unauthorized Immigration to the United States," Annual Review of Sociology 21 (1995): 195-216; Doug S. Massey and Audrey Singer, "New Estimates of Undocumented Mexican Migration and the Probability of Apprehension," Demography 32, no. 2 (May 1995): 203-13; and "Estimates of the Unauthorized Immigrant Population Residing in the United States: 1990 to 2000" (Washington: Office of Policy and Planning, U.S. Immigration and Naturalization Service), https://www.dhs.gov/xlibrary/assets/statistics/publications/Ill_Report_1211.pdf.

37. U.S. General Accounting Office, "Overstay Tracking: A Key Component of Homeland Security and a Layered Defense," GAO-04-82 (Washington: GAO, May 2004), http://www.gao.gov/new.items/d0482.pdf.

38. Combining three sources for three time periods yields an astonishing 50,052,500 illegal entries: (1) the estimated gross illegal entries from Massey and Singer, "New Estimates," for the years 1975 to 1989; (2) Robert Warren and Donald Kerwin, "Beyond DAPA and DACA: Revisiting Legislative Reform in Light of Long-Term Trends in Unauthorized Immigration to the United States," Journal on Migration and Human Security 3, no. 1 (2015), for the years 1990 to 2009; and (3) estimating from Jeffrey S. Passel and D'Vera Cohn, "Trends in Unauthorized Immigration: Undocumented Inflow Now Trails Legal Inflow" (Washington: Pew Research Center, 2008), for the years 2009 to 2015.

39. "San Bernardino Shooting," CNN, http://www.cnn.com/specials/san-bernardino-shooting.

40. Illegal immigrants are not really a visa category, but they are listed as such for simplicity's sake.

41. No discount rate adjustment.

42. U.S. Citizenship and Immigration Services, "Lawful Permanent Resident (LPR)" (Washington: DHS), https://www.uscis.gov/tools/glossary/lawful-permanent-resident-lpr.

43. Bureau of Consular Affairs, "Student Visa" (Washington: U.S. Department of State), https://travel.state.gov/content/visas/en/study-exchange/student.html.

44. F and M visas are for students.

45. Wadih el-Hage was on a student visa when he and Glen Cusford Francis likely assassinated Dr. Rashad Khalifa on January 31, 1990, in Tucson, Arizona.

46. Bureau of Consular Affairs, "Nonimmigrant Visa for a Fiancé(e) (K-1)" (Washington: U.S. Department of State), https://travel.state.gov/content/visas/en/immigrate/family/fiance-k-1.html.

47. Alex Nowrasteh, "Secret Policy to Ignore Social Media? Not So Fast," Cato at Liberty, December 15, 2015, https://www.cato.org/blog/secret-policy-ignore-social-media-not-so-fast.

48. U.S. Citizenship and Immigration Services, "Refugees" (Washington: DHS), https://www.uscis.gov/humanitarian/refugees-asylum/refugees.

49. Charlotte J. Moore, "Review of U.S. Refugee Resettlement Programs and Policies," Congressional Research Service (Washington: Government Printing Office, March 1, 1981), pp. 3-16.

50. Matthew Hendley, "Paul Gosar Thinks Abdullatif Aldosary, Alleged Bomber, Is a ‘Known Terrorist'; He Is Not," Phoenix New Times, December 7, 2012, http://www.phoenixnewtimes.com/news/paul-gosar-thinks-abdullatif-aldosary-alleged-bomber-is-a-known-terrorist-he-is-not-6647318.

51. U.S. Government Accountability Office, "Combating Terrorism: Foreign Terrorist Organization Designation Process and U.S. Agency Enforcement Actions," GAO-15-629 (Washington: GAO, June 2015), http://www.gao.gov/assets/680/671028.pdf.

52. U.S. Citizenship and Immigration Services, "The United States Refugee Admissions Program (USRAP) Consultation & Worldwide Processing Priorities" (Washington: DHS), https://www.uscis.gov/humanitarian/refugees-asylum/refugees/united-states-refugee-admissions-program-usrap-consultation-worldwide-processing-priorities.

53. Bureau of Consular Affairs, "Visitor Visa" (Washington: U.S. Department of State), https://travel.state.gov/content/visas/en/visit/visitor.html.

54. . Bureau of Consular Affairs, "Visa Waiver Program" (Washington: U.S. Department of State), https://travel.state.gov/content/visas/en/visit/visa-waiver-program.html#reference.

55. Ibid.

56. Steven A. Camarota, "The Open Door: How Military Islamic Terrorists Entered and Remained in the United States, 1993-2001," Center for Immigration Studies, Center Paper no. 21, May 2002, http://cis.org/sites/cis.org/files/articles/2002/theopendoor.pdf.

57. Office of Inspector General, "An Evaluation of the Security Implications of the Visa Waiver Program," OIG-04-26 (Washington: DHS, April 2004), pp. 11-12, https://www.oig.dhs.gov/assets/Mgmt/OIG_SecurityImpVisaWaiverProgEval_Apr04.pdf.

58. Two terrorists killed one person in an attack, so they each got credit for one-half of the murder.

59. DHS, "The Strategic National Risk Assessment in Support of PPD 8."

60. Camarota, "The Open Door: How Military Islamic Terrorists Entered and Remained in the United States, 1993-2001."

61. Max Abrahms, "Why Terrorism Does Not Work," International Security 31, no. 2 (Fall 2006): 42-78.

62. Kudlow, "I've Changed"; Bossie, "Conservatives Should Think Bigger"; and Ann Coulter, interview by Breitbart News Saturday.

63. See Mueller and Stewart, "Evaluating Counterterrorism Spending."

64. Mueller and Stewart, Chasing Ghosts, p. 188.

65. Benjamin Powell, "Coyote Ugly: The Deadweight Cost of Rent Seeking for Immigration Policy," Public Choice 150 (2012): 195-208.

66. George Borjas, "Immigration and the American Worker: A Review of the Academic Literature" (Washington: Center for Immigration Studies, April 2013), p. 2, http://cis.org/immigration-and-the-american-worker-review-academic-literature.

67. World Travel and Tourism Council, "Travel & Tourism: Economic Impact 2015, United States of America," London, World Travel and Tourism Council, p. 5, http://www.wttc.org/-/media/files/reports/economic%20impact%20research/countries%202015/unitedstatesofamerica2015.pdf.

68. Mueller and Stewart, "Evaluating Counterterrorism Spending," pp. 239-40.