The worst terrorist attack in world history occurred on September 11, 2001. Many security failures contributed to permitting the attacks to occur, including the inability of the legal immigration system to stop the 19 hijackers from receiving temporary nonimmigrant visas to travel to the United States. Fortunately, the system has improved, and vetting failures have become far less frequent after the attacks. 

In 2018, the Cato Institute published my original research documenting terrorism visa-vetting failures from 1987 to 2016. The report demonstrated for the first time that visa vetting failures became much less common in the years following 9/11—despite the US government’s even more aggressive foreign policy and far more legal immigration and travel than before 9/11. 

As defined in the report, a visa vetting failure occurs when someone goes on to commit a terrorism offense after entering the United States with views supporting terrorism, regardless of whether they were manifested in any public way. I used this broad definition because, theoretically, the visa vetting process could uncover even privately held views. 

Of the 531 individuals killed while committing a terrorist attack or convicted of a terrorism offense—including sending money to groups abroad—from 2002 to 2016, 13 (2.45 percent) were in the United States as a result of post‑9/​11 visa vetting failures. This compares to 52 pre‑9/​11 vetting failures. The vetting system permitted the entry of one radicalized terrorist for every 29 million visa or status approvals—a rate 84 percent below the pre‑9/​11 rate. 

This means that improved visa vetting would have had no effect on 97.5 percent of terrorism offenders since 9/11 because they entered before the 9/11 improvements, were born in the United States, or were immigrants who were not vetting failures, according to government sources. Most of these immigrants entered as children or otherwise radicalized after entry, while others were extradited here or deliberately lured here as part of sting operations. Of the 13 post‑9/​11 vetting failures, only one (Tashfeen Malik in San Bernardino) killed anyone, compared to the 27 deadly failures before 9/11, meaning that deadly visa vetting failures were reduced by 96 percent. 

Malik and her American accomplice killed 14 people. If we attribute all 14 to Malik, the chance of a US resident (American or immigrant) being killed in an attack committed by a terrorist who entered as a result of a vetting failure from 2002 to 2016 was about 1 in 328 million per year. This rate was 99.99 percent lower than the risk of normal homicides (1 in 20,000 per year). It was also 99.5 percent lower than the risk of being killed as a result of visa vetting failure before 9/11. 

My colleague Alex Nowrasteh has published research about how the risk of foreign-born terrorism generally—from vetting failures or from post-entry radicalization—was also much lower than in the period from 1975 to 2001, including 9/11 itself. For the full period from 1975 to 2024, the chance of being killed by a foreign-born terrorist in the United States was just 1 in 4.6 million per year, but since 9/11, the risk was just one in 165 million. 

Using Nowrasteh’s research, only one additional deadly attacker since 2016 would qualify as a vetting failure: Saudi-born Mohammed Saeed Alshamrani, who entered in 2017 and murdered three people in Florida in 2019. His case is unique in that the Pentagon sponsored him to come to the United States to train as a pilot so that he could fly planes that the United States sold to the Saudi government as part of an arms deal. This means that the only deadly visa vetting failure in the last decade occurred thanks in part to the Trump administration’s sponsorship.

In the report, I discuss the many ways in which the vetting system improved to reduce the rate of terrorism vetting failure. Among other things, officials at ports of entry and consular posts received better training; surveillance of foreign terrorist groups increased; foreign countries entered into surveillance-sharing agreements; and government databases were better integrated into the legal immigration vetting process. 

This research doesn’t mean that there will never be another failure, nor does it suggest that the government should not be concerned about preventing terrorism travel, but it does show that its targeted efforts since 9/11 have worked. There is no need for blanket bans like those the current administration is imposing today. 

The case for broad-based immigration restrictions based on terrorism is the weakest that it has ever been. Terrorism generally, including foreign-born terrorism specifically, is a small risk to the public, but legal immigrants generally and legal immigrants from Middle Eastern countries specifically are about 75 percent less likely to commit crimes for which they are incarcerated in the United States. By lowering the rate of crime, these immigrants overall improve public safety from the far more common violent crimes. Immigrant terrorists—including those on 9/11—accounted for just 0.3 percent of US murders from 1975 to 2024, so lowering the overall rate of violence is much more important.

Unfortunately, as I detailed in a paper for The Independent Review, the most lasting immigration legacy of 9/11 is how it derailed bipartisan efforts to improve the legal immigration system for peaceful, productive immigrants, students, and travelers. As the rest of the world became more open to legal immigration, the United States stood alone in resisting any efforts to update its flailing processes to let more vetted immigrants come to the United States—from anywhere.