WASHINGTON — Next time you go to the airport, there may be more eyes on you than you notice.

Specially trained security personnel are watching body language and facial cues of passengers for signs of bad intentions. The watcher could be the attendant who hands you the tray for your laptop or the one standing behind the ticket-checker. Or the one next to the curbside baggage attendant.

They're called Behavior Detection Officers, and they're part of several recent security upgrades, Transportation Security Administrator Kip Hawley told an aviation industry group in Washington last month. He described them as "a wonderful tool to be able to identify and do risk management prior to somebody coming into the airport or approaching the crowded checkpoint."

The officers are working in more than a dozen airports already, according to Paul Ekman, a former professor at the University of California at San Francisco who has advised Hawley's agency on the program. Amy Kudwa, a TSA public affairs specialist, said the agency hopes to have 500 behavior detection officers in place by the end of 2008.

Kudwa described the effort, which began as a pilot program in 2006, as "very successful" at identifying suspicious airline passengers. She said it had netted drug carriers, illegal immigrants and terrorism suspects. She wouldn't say more.

At the heart of the new screening system is a theory that when people try to conceal their emotions, they reveal their feelings in flashes that Ekman, a pioneer in the field, calls "micro-expressions." Fear and disgust are the key ones, he said, because they're associated with deception.

Behavior detection officers work in pairs. Typically, one officer sizes up passengers openly while the other seems to be performing a routine security duty. A passenger who arouses suspicion, whether by micro-expressions, social interaction or body language gets subtle but more serious scrutiny.

A behavior specialist may decide to move in to help the suspicious passenger recover belongings that have passed through the baggage X-ray. Or he may ask where the traveler's going. If more alarms go off, officers will "refer" the person to law enforcement officials for further questioning.

The strategy is based on a time-tested and successful Israeli model, but in the United States, the scrutiny is much less invasive, Ekman said. American officers receive 16 hours of training — far less than their Israeli counterparts_ because U.S. officials want to be less intrusive.

The use of "micro-expressions" to identify hidden emotions began nearly 30 years ago when Ekman and colleague Maureen O'Sullivan began studying videotapes of people telling lies. When they slowed down the videotapes, they noticed distinct facial movements and began to catalogue them. They were flickers of expression that lasted no more than a fraction of a second.

The Department of Homeland Security hopes to dramatically enhance such security practices.

Jay M. Cohen, undersecretary of Homeland Security for Science and Technology, said in May that he wants to automate passenger screening by using videocams and computers to measure and analyze heart rate, respiration, body temperature and verbal responses as well as facial micro-expressions.

Homeland Security is seeking proposals from scientists to develop such technology. The deadline for submissions is Aug. 31.

The system also would be used for port security, special-event screening and other security screening tasks.

It faces high hurdles, however.

Different cultures express themselves differently. Expressions and body language are easy to misread, and no one's catalogued them all. Ekman notes that each culture has its own specific body language, but that little has been done to study each individually in order to incorporate them in a surveillance program.

In addition, automation won't be easy, especially for the multiple variables a computer needs to size up people. Ekman thinks people can do it better. "And it's going to be hard to get machines that are as accurate as trained human beings," Ekman said.

Finally, the extensive data-gathering of passengers' personal information will raise civil-liberties concerns. "If you discover that someone is at risk for heart disease, what happens to that information?" Ekman asked. "How can we be certain that it's not sold to third parties?"

Whether mass-automated security screening will ever be effective is unclear. In Cohen's PowerPoint slide accompanying his aviation industry presentation was this slogan: "Every truly great accomplishment is at first impossible."