News that incautious comments on “tweeter” got British tourists excluded from the United States had Twitter alight yesterday. (Paperwork given to one of the two, on display in this news story, refers to the popular social networking site as a “Tweeter website account,” betraying some ignorance of what Twitter is.)

It’s a good chance to review how suspicion is properly—and, here, improperly—generated.

The Department of Homeland Security has been vague as yet about what actually happened. It may have been some kind of “social media analysis” like this that turned up “suspicious” Tweets leading to the exclusion, though the betting is running toward a suspicious-activity tipline. (What “turned up” the Tweets doesn’t affect my analysis here.) The boastful young Britons Tweeted about going to “destroy America” on the trip—destroy alcoholic beverages in America was almost certainly the import of that line—and dig up the grave of Marilyn Monroe.

Profoundly stilted literalism took this to be threatening language. And a failure of even brief investigation prevented DHS officials from discovering the absurdity of that literalism. It would be impossible to “dig up” Marilyn Monroe’s body, which is in a crypt at Westwood Memorial Park in Los Angeles.

I testified to the Senate Judiciary Committee in 2007 about how one might mine data for terrorists and terrorism planning, in terms that apply equally well to Twitter banter and to any criminality or wrongdoing. For valid suspicion to arise, the information collected must satisfy two criteria:

(1) It is consistent with bad behavior, such as terrorism planning or crime; and (2) it is inconsistent with innocent behavior. In … the classic Fourth Amendment case, Terry v. Ohio, …  a police officer saw Terry walking past a store multiple times, looking in furtively. This was (1) consistent with criminal planning (“casing” the store for robbery), and (2) inconsistent with innocent behavior — it didn’t look like shopping, curiosity, or unrequited love of a store clerk. The officer’s “hunch” in Terry can be described as a successful use of pattern analysis before the age of databases.

Similarly, using the phrase “destroy America” is consistent with planning to destroy America. (You want to be literal? Let’s be literal!) But it’s also consistent with talking smack, which is innocent behavior. These Tweets fail the second criterion for generating suspicion.

Twitter is nothing if not an unreliable source of people’s thinking and intentions. It’s a hotbed of irony, humor, and inside jokes. Witness this Tweet of mine from yesterday, which failed to garner the social media guffaw I sought (which is why I link to it here). Things said on Twitter will almost never be suspicious enough to justify even the briefest interrogation.

Other facts could combine with Twitter commentary to create a suspicious circumstance on extremely rare occasions, but for proper suspicion to arise, the Tweet or Tweets and all other facts must be consistent with criminal planning and inconsistent with lawful behavior. No information so far available suggests that the DHS did anything other than take Tweets literally in the face of plausible explanations by their authors that they were using hyperbole and irony. This is simple investigative incompetence.

If indeed it is a “social media analysis” program that produced this incident, the U.S. government is paying money to cause U.S. government officials to waste their time on making the United States an unattractive place to visit. That’s a cost-trifecta in the face of essentially zero prospect for any security benefit. I slept no more soundly last night knowing that some Brits were denied a chance to paint the town red in L.A.

In case it needs explaining, “paint the town red” is archaic slang. It does not imply an intention or plan to apply pigments to any building or infrastructure in Los Angeles, whether by brush, roller, or spray can.