The reliability of the Transportation Security Administration's program to weed out terrorists based on their behavior among travelers is coming under scrutiny. Doubts about the program are coming directly from within the TSA, according to documents the ACLU obtained from the agency via the Freedom of Information Act.

The ACLU report (PDF) says that the TSA's own files were loaded with research questioning the behavior detection program. The program has cost taxpayers more than $1.5 billion to deploy 3,000 detection officers at 176 airports nationwide over the last decade.

"Academic research and other documents in the TSA's own files reinforce that behavior detection is unscientific and unreliable," the ACLU said. "The TSA repeatedly overstated the scientific validity of behavior detection in communications with members of Congress and the Government Accountability Office."

In response, the TSA defended the Screening Passengers by Observation Program, which is meant to single out terrorists at airports by their actions and verbal clues.

“TSA’s behavior detection approach is designed to identify and engage individuals who may be high-risk (e.g., possess malicious intent) on the basis of an objective process using behavioral indicators and thresholds, and then route them to additional security screening," TSA said in a statement. "It is one element of TSA’s efforts to mitigate threats against the traveling public and is critical to TSA’s systems approach to deter, detect, and disrupt individuals who pose a threat to aviation."

The ACLU report, which highlights allegations that the program led to racial profiling, comes four years after the General Accountability Office found defects in the agency's studies validating the program. At the time, a review of about 400 studies on the issue concluded that the chances of spotting deceptive behavior was about in line with flipping a coin.

During 2011 and 2012, according to that GAO report, detection officers in uniform or in plain clothes referred about 8,700 travelers at 49 airports to law enforcement officers. That led to 365 arrests, the bulk in connection to drug and immigration violations—but none for terrorism.