Writing in Slate on Thursday, political reporter David Weigel identified a “full-blown revolt against the TSA.” On Saturday, would-be passenger John Tyner refused an airport security pat-down with the now-famous phrase “if you touch my junk I’m going to have you arrested“; Texas congressman Ron Paul introduced the American Traveler Dignity Act to establish “that airport security screeners are not immune from any US law”; and several groups have designated Wednesday, Nov. 24 (the day before Thanksgiving), National Opt-Out Day against invasive body scanners. According to the TSA’s Web site, new security measures like full-body scanners are just part of its mission “to prevent any terrorist or criminal activity“—but have TSA screeners ever actually prevented a terrorist attack?

It’s hard to say. The TSA was unable to provide any comprehensive data covering all nine years of its existence on short notice, but it does publicize incidents on a weekly basis: From Nov. 8 to Nov. 14, for example, agents found six “artfully concealed prohibited items” and 11 firearms at checkpoints, and they arrested six passengers after investigations of suspicious behavior or fraudulent travel documents. (Those figures are close to the weekly average.) It’s not clear, however, whether any of these incidents represent attempted acts of terrorism or whether they were honest accidents. (Whoops, forgot I had that meat cleaver on me! Or, I had no idea flares weren’t allowed!)

Citing national-security concerns, the TSA will not point to any specific cases in which a screener stopped a would-be terrorist at a checkpoint. Nonaffiliated security experts, such as Bruce Schneier (who coined the term “security theater“), argue that that’s because this has never happened. It’s true the TSA doesn’t make a habit of keeping success stories a secret. In April 2008, the TSA touted the arrest of U.S. Army veteran Kevin Brown at Orlando International Airport as a victory for its behavioral detection program. Brown was arrested after trying to check luggage containing pipe-bomb-making materials. Airline officials insisted passengers were never in danger, since Brown didn’t intend to assemble the bomb on the plane. Moreover, he did not have ties to organized terrorism, and it’s not apparent what he wanted to do with the hazardous materials after arriving at his destination. Brown fits into the category of troublemakers that Schneier says the TSA does catch: random nut jobs. (Not professional terrorists with thought-out plans.)

The aforementioned “behavioral detection program,” also known as SPOT (Screening of Passengers by Observational Techniques), has been one of the TSA’s most roundly criticized initiatives. In May, the Government Accountability Office released a report noting that SPOT’s annual cost is more than $200 million and that as of March 2010 some 3,000 behavior detection officers were deployed at 161 airports but had not apprehended a single terrorist. (Hundreds of illegal aliens and drug smugglers, however, were arrested due to the program between 2004 and 2008.) What’s more, the GAO noted that at least 16 individuals later accused of involvement in terrorist plots flew 23 different times through U.S. airports since 2004, but TSA behavior-detection officers didn’t sniff out any of them.

What these numbers don’t get at is whether the TSA airport screeners prevent terrorist attacks through their very existence—deterring plots by hanging around. This is quite probably the case, but it’s not obvious that they prevent any more attacks than the private contractors who handled checkpoints before the Aviation and Transportation Security Act of 2001 went into effect.

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Explainer thanks Lauren Gaches of the Transportation Security Administration and Bruce Schneier.

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