WASHINGTON — Looking for signs of "stress, fear and deception" among the hundreds of passengers shuffling past him at Orlando (Fla.) International Airport one day last month, security screener Edgar Medina immediately focused on four casually dressed men trying to catch a flight to Minneapolis.

One of the men was giving obvious signs of trying to hide something, Medina said. After obtaining the passengers' ID cards and boarding passes, the Transportation Security Administration officer quickly determined the men were illegal immigrants traveling with fake Florida driver's licenses. They were detained.

"It wasn't that unusual," Medina said. "We see more and more of that stuff down here. Every day, that is what I'm looking for."

The otherwise mundane arrests Aug. 13 illustrated an increasingly popular tactic in the government's effort to fight terrorism: detecting lawbreakers or potential terrorists by their behavior. The TSA has embraced the strategy, training 600 of its screeners in detection techniques. By year's end, 1,000 screeners at more than 40 airports will be trained.

40,000 PICKED OUT

Although civil libertarians and top Democrats in Congress say the techniques raise serious questions about privacy rights and racial and ethnic profiling, TSA officials say the behavior-detection officers may play a more important role in thwarting terrorist attacks than traditional screening techniques.

The teams have referred more than 40,000 people for extra screening since January 2006. Of those passengers, nearly 300 were arrested on charges including carrying concealed weapons and drug trafficking. TSA officials will not say whether the screeners have helped nab potential terrorists but say terrorists and other lawbreakers exhibit the same behavioral clues.

"In this kind of environment, you can't be sure they are going to come to the checkpoint with a prohibited item, per se," said Kip Hawley, TSA's administrator. "Unless you do something more than that, you are going to miss the next attack. A behavior-detection officer will detect somebody no matter what the weapon is."

The TSA's teams are the most publicly acknowledged effort by the government or the private sector to come up with strategies and technology to detect lawbreakers or terrorists before they commit a crime. Other technologies under development or being deployed include machines that detect stress in voices and software that scans video images to match the faces of passengers with those of known terrorists.

The government is testing other technology that can see through clothing with Superman-like vision or can help determine whether somebody might be carrying an explosives-laden vest by analyzing electromagnetic waves. Some of that technology has already been deployed at some airports and in mass transit terminals.

CONCEALED EMOTIONS

To become a behavior-detection officer, screeners undergo four days of classroom training and three days of supervised on-the-job work.

A new tool in their arsenal is the ability to determine when the slightest facial movement is masking a lie. All of TSA's behavior-detection officers and the agency's 1,000 inspectors, who also work at airports, will be trained in the technique.

David Matsumoto, research director for the Ekman Group, which conducts the TSA "micro-facial expression" training, said that micro-expressions are signs of concealed emotions and "are indications that the travelers have an emotional state that they don't want anyone else to know about."

The TSA's growing reliance on detecting behavior and the close study of passengers' expressions concerns civil liberties groups and members of Congress.

"The problem is behavioral characteristics will be found where you look for them," said John Reinstein, legal director of the American Civil Liberties Union of Massachusetts.

Rep. Bennie Thompson, D-Miss., chairman of the House Homeland Security Committee, said the program's aggressive expansion is a concern. He plans to hold hearings on the issue in the next few months, he said.

"We have to be careful in using this so we don't single out people who look different than us," Thompson said. "When we get into something that is approaching behavior, we have to be very careful that we don't stereotype people because of their dress or their race. And we have to understand and protect the civil liberties and civil rights of people in this country."