(CBS News) WASHINGTON -- At Washington's Union Station, CBS News watched two men with backpacks as they watched everyone else. They are undercover behavior detection officers, trained to pick out bad guys by looking for expressions of fear, anxiety or deception.
Since 2007, the Transportation Security Administration has spent $900 million training and deploying 3,000 behavior detection officers. Those so-called BDOs are now stationed at major transit hubs, primarily at 176 of the nation's busiest airports.
But a new report from the Government Accountability Office questions whether BDOs are effective, concluding: "Available evidence does not support whether behavioral indicators ... can be used to identify persons who may pose a risk to aviation security."
The TSA argues the BDOs provide a critical layer of defense by looking for signals which can't be picked up by metal detectors or explosives scanners.
In a statement, the TSA said: "Looking for suspicious behavior is a common sense approach ... and when combined with other security layers helps mitigate a variety of threats."
A 2011 Homeland Security analysis found the BDOs to be highly effective, identifying high-risk passengers far more often than random screening.
But the GAO study rejected that Homeland study, saying the findings were based on bad science. And the GAO questions the whole premise of behavior-based detection, concluding it "is the same or slightly better than chance."
The GAO, with some support in Congress, now is calling on Homeland Security to cut funding for the behavioral program until it can be proven effective. But TSA is promising to fight to keep the BDOs at their posts.