The U.S. Transportation Security Administration has long pursued a far-fetched belief that its agents can be trained to spot potential terrorists simply by observing the passengers thronging the nation’s airports.

This idea, unsupported by any reliable evidence, has led the TSA to spend nearly $1 billion dollars over the past six years to train and deploy nearly 3,000 agents who wander around the nation’s 176 largest airports looking for people exhibiting signs of stress.

Despite repeated studies by the Government Accountability Office and the Transportation Department’s Inspector General saying the project is useless, the agency’s top management clings to what it cutely calls Screening of Passengers by Observation Technique, or SPOT.

Last week, the GAO recommended stopping the ineffective program — a move that would save $200 million a year.

As the GAO’s thorough, 99-page report put it: “Until TSA can provide scientifically validated evidence demonstrating that behavioral indicators can be used to identify passengers who may pose a threat to aviation security, the agency risks funding activities that have not been determined to be effective.”

Yet a TSA statement persisted in defending its repeatedly discredited practice: “Behavior detection is vital to TSA’s layered approach to deter, detect, and disrupt individuals who pose a threat to aviation. Looking for suspicious behavior is a common sense approach used by law enforcement and security personnel across the country and the world.”

Though the program has led to a few random arrests of wanted domestic criminals, it hasn’t caught any terrorists.

And the 2010 Times Square bomber, attempting to flee the country, passed through security at New York City’s JFK airport. He was only apprehended once he was aboard an aircraft. The GAO determined that agents’ ability to correctly assess suspicious behavior was scarcely better than random.

TSA Administrator John Pistole admitted as much last week under strong cross-examination by 1st District Rep. Mark Sanford, R-S.C., during a congressional hearing.

Rep. Sanford asked how TSA agents “get into somebody else’s head?”

He pressed further for answers on how a TSA agent could distinguish between the nervous energy exhibited by someone with a criminal record, a woman fleeing an abusive husband, a believer in government conspiracy or an illegal immigrant.

Sanford reasonably wanted to know whether these individuals exhibit the “stress and fear” that the agent was looking for.

To each example, Mr. Pistole answered with a variation of “It depends on the individual, but potentially, sure.”

Rep. Sanford rightly responded: “Which I think raises the point which the GAO report has brought.”

Rep. Sanford also asked why, with all the other security features in place at airports, does TSA “have to go through a screening process based on somebody’s interpretation of what might be in your brain?”

The result, the congressman fairly concluded, is “a billion dollars [spent] with no results.”

And the result of this GAO report should be congressional action to stop this waste of taxpayer money.