WASHINGTON -- A federal probe of a Transportation Security Administration program to screen suspicious behaviour of passengers at airports suggests the effort, which has cost almost $1 billion since 2007, has not been proven effective, according to a report released Wednesday.

The Government Accountability Office said its investigation found that the results of the TSA program -- called Screening of Passengers by Observation Techniques -- were "no better than chance." Under the program, agents identify suspicious looking people and talk to them to determine whether they pose a threat. The investigators looked at the screening program at four airports, chosen on the basis of size and other factors.

"TSA has yet to empirically demonstrate the effectiveness of the program despite spending about $900 million on it since 2007," said Steve Lord, who directed the investigation for the GAO. He said the GAO, which is the research and investigative arm of Congress, "conducts active oversight of the TSA for the Congress given their multibillion-dollar budget." He said "the behaviour detection program is viewed as a key layer of aviation security."

The investigation found that behaviour detection officers at the four airports said some behavioural indicators they used were "subjective." The report said the TSA is still collecting evidence on the screening program but has indicated it needs more time to determine the program's effectiveness.

Both Republican and Democratic lawmakers on the House Homeland Security Committee said the report raises serious questions about the screening program.

"While I believe there is value in utilizing behaviour detection and analysis in the aviation environment, especially since it is used successfully by law enforcement, we can only support programs that are proven effective," said Rep. Mike McCaul, R-Texas, the committee's chairman.

The leading Democrat on the committee also voiced concerns.

Rep. Bennie Thompson, D-Miss., said the GAO report found that the "program is fundamentally flawed, cannot be proven effective and should no longer be funded with taxpayer dollars."

Committee members will consider the report's findings at a subcommittee hearing Thursday.