NEW YORK — To the untrained eye, the man looked like any other traveler as he waited in line at Kennedy Airport. But something about the way he was acting caught the attention of two security screeners.

For 16 minutes, they questioned him, scanned every inch of his body twice with a metal-detecting wand and emptied his carry-on bag onto a table. Out came a car stereo with wires dangling from it.

The man was eventually found to have done nothing wrong — he said he had pulled the stereo out of his car because he was afraid it would get stolen — and he was sent on his way.

But it's the type of scene that has unfolded on a regular basis over the past four years at the nation's major airports under a rapidly expanding "behavior detection" program set up by the Transportation Security Administration to spot terrorists or other dangerous air travelers by way of subtle clues in the way they act.

The agency's efforts drew attention this week when screeners trained in behavior detection in Orlando arrested an Army veteran after he tried to check luggage containing pipe bomb-making materials onto a flight to Jamaica.

But that collar was something of a rarity. In the four years since the program was launched, the TSA has yet to encounter any would-be suicide bombers. The most common catches have been people carrying fake IDs.

SUSPICIOUS BEHAVIOR

Of the more than 104,000 air travelers who were plucked out of security lines and subjected to a more intense level of screening because of something suspicious in their demeanor, fewer than 700 were ultimately arrested, officials said.

Many more — about 9,300 — revealed something during the screening process that caused the TSA to call in law enforcement for a more thorough investigation.

About half of those passengers weren't suspected of any particular crime, but behaved suspiciously enough that screeners thought police should be called anyway. More than half of the other referrals involved people carrying fraudulent documents, the TSA said. A small percentage involved drugs, contraband currency, immigration violations, or discoveries that a passenger was wanted by police.

Dubbed the SPOT program, for Screening Passengers by Observation Techniques, the effort is shrouded in secrecy that makes it difficult to evaluate its worth.

TSA officials refuse to say exactly what sort of behavior can make them suspicious, but part of the effort relies on watching for fleeting facial expressions that indicate a person is under stress and has something to hide. Behavior agents also casually question people about where they are headed and look for clues in their responses.

Federal officials said the program, which requested a $45 million budget this year, is a worthwhile complement to random searches and an alternative to racial profiling.

'JUST FOR SHOW'

But the program has its doubters.

Barry Steinhardt of the American Civil Liberties Union said the TSA has released too little information about its behavioral analysis techniques to assess whether the program works, or is "just for show."

"Whether this is anything more than profiling under another name, we don't know," he said.

The TSA began experimenting with behavior agents in Boston nearly five years ago, in part because of the perceived success of a similar program in Israel.

Today, a variety of security consultants offer training in various methods of deception detection, including University of California-San Francisco professor Paul Ekman and Rafi Ron, former security director at Israel's Ben-Gurion International Airport.

Most passengers who are pulled aside for extra questioning and a search of their carry-on luggage are allowed to continue to their flights, and almost none are ever told why they were stopped.

Unlike police officers, who do not have the right to stop someone without cause on the street, TSA agents are legally allowed to thoroughly search someone trying to board a plane and interrogate them at length, even if there is no evidence they have broken any law.

Homeland Security officials are pleased enough with the results that they plan to increase the number of behavioral detection officers substantially in the coming months. Today, there are about 1,200 of the agents at 70 large airports. That number is expected to double to 2,400 at 160 airports by September, and grow to 4,000 by mid-2009.

DECEPTION DETECTION

Michigan State University professor Timothy Levine, who studies deception detection, said scientists are split over whether it is possible to train people to recognize terrorist operatives or nervous criminals by observing their demeanor.

"I'm a skeptic," Levine said. "There are a lot of reasons for people to be emotional or aroused, other than deception. Especially at airports."

He said his own research has suggested that people do commonly offer up behavioral clues when they are trying to hide something. "But they aren't big. They are subtle and they vary tremendously, by situation, people and context," he said.

Levine added, though, that the program might still be worth a shot.

"Maybe it wouldn't catch the smooth operative," he said. But even a poorly trained agent, he said, might have luck catching someone like "shoe bomber" Richard Reid.

The TSA invited The Associated Press to Kennedy Airport late last month to watch two of its agents, James Rivera and Pat Marcoux, at work.

The pair said that, over the years, they have grabbed people trying to carry huge amounts of currency through customs without reporting it, and seen all manner of strange items stashed in people's luggage, including a book infested with roaches.

"There's always a reason why you're exhibiting that behavior that catches our attention," Rivera said. "Maybe it's just because you're having problems at home."

It is difficult, even while watching behavior detection officers work, to assess just what type of behavior triggers their interest. Their style seemed deliberately low-key.

AGENTS NEED 'ANSWERS'

Working quietly in tandem, Rivera and Marcoux stopped one pair of smiling young men after they passed through the metal detectors and chatted them up for about 10 minutes while they searched their luggage. They were eventually allowed to continue to their plane.

The man with the car radio was singled out for tougher screening before he had even put his bag on the belt for the X-ray machine. But Rivera and Marcoux would not say what raised their suspicions.

Besides scanning him with a wand, and running hands along the outside of his legs to check for weapons, the agents handled his clothing, flipped through a book in his bag and questioned him about the purpose of his trip.

In the end, agents got answers that explained why the man had seemed out of sorts. They learned that he was traveling to the Dominican Republic to visit a wife he hadn't seen in a year and was a little anxious about the trip.

They checked his ID and let him go. He left with a handshake and a smile.

After all, Marcoux said, "there is a customer service aspect to the job."

"People are stressed enough already. They don't need us to escalate the situation," Rivera said.