Washington (CNN) -- Ten years after the Transportation Security Administration started a program to spot terrorists by observing passenger behavior, the agency has spent more than $900 million on the program but has yet to prove its effectiveness, congressional overseers say.
And Congress risks wasting more money if it continues funding the program without scientific proof that it works, the Government Accountability Office said in a report.
The report is the most critical yet pitting the GAO critics against the TSA, which has long believed that trained observers can pick out people who pose a threat to aviation by looking for signs of stress, fear or deception.
The agency has deployed an ever-increasing number of so-called behavior detection officers, calling them a "vital component" of its security program.
In fiscal 2011 and 2012, about 3,000 such officers were deployed to 176 major airports nationwide, where they observed about 1.3 billion people, the report says.
The GAO said TSA tests of the program were flawed, and decades of published research on behavior detection "also draw into question the scientific underpinnings" of the program. Those studies show that the ability of trained observers to detect deceptive behavior is "the same as or slightly better than chance."
But the TSA defends the program, saying security would be damaged if money is cut. "Behavior detection techniques have been an accepted practice for many years within the law enforcement, customs and border enforcement, defense, and security communities both in the United States and internationally," the TSA wrote in a response to the GAO.
Under the program, uniformed behavior detection officers scan passengers in line and engage them in brief verbal exchanges, identifying passengers who exhibit certain behaviors. Different behaviors are assigned point values, and if a passenger exceeds a point "threshold," they are referred to secondary screening, where they undergo a pat-down and search of their personal property, while officers continue to look for behavioral clues.
If another threshold is passed, or if fraudulent documents are discovered, the behavior detection officer refers the passenger to a law enforcement officer for further investigation. The law enforcement officer may choose to allow the passenger to proceed or could question the passenger further.
The initial observation takes less than 30 seconds, but the lengthier screening takes an average 13 minutes, and referrals to police can take longer, according to an outline of the process in the GAO report.
"The report released today by GAO displays what I have been saying for years -- that TSA's (detection) program is fundamentally flawed, cannot be proven effective, and should no longer be funded with taxpayers dollars," Rep. Bennie Thompson, D-Mississippi, said in a written statement.
Thompson, the ranking Democrat on the House Homeland Security Committee, said he will question TSA Administrator John Pistole about the report at a hearing Thursday.