When someone at an airport is sweating, is it because he's running late or trying to hide something? Could hand signals between people in a terminal be part of an inside joke or a terror plot?
A pilot program using "behavior pattern recognition" is under way at Boston's Logan International Airport, where two of the planes used by the Sept. 11 hijackers took off. Air marshals, passenger screeners and state police stationed there have undergone special training in things to look for that could indicate a terrorist plot.
Israeli officials have employed a version of the technique for years to protect air travelers against terrorists.
At Logan, uniformed and undercover security officials watch people as they move through terminals. They look for odd or suspicious behavior: heavy clothes on a hot day, loiterers without luggage, anyone observing security methods.
At the security checkpoints, screening supervisors have a score sheet with a list of behaviors on it. If a passenger hits a certain number, a law enforcement officer will be notified to question the person.
Air marshals watch the airport crowds as they wait for their flights; they, too, alert the troopers if they see something suspicious.
"They're looking for something outside the normal range of behavior," said Jack Shea, special agent in charge of the federal air marshals in Boston. "What I like about it, it's very basic, it's common sense, it's effective, it works."
Massachusetts State Police Maj. Tom Robbins, who oversees the troopers at Logan, said the program has been a success.
"We haven't caught Osama bin Laden, but we've caught people who are exhibiting the behaviors they're looking for," he said. Mostly, they catch people with outstanding warrants, he said.
The technique is not new to the government - Customs agents have used the technique to look for smugglers at border crossings. But some civil libertarians are wary, concerned the program could lead to unlawful searches and seizures and persecution of law-abiding passengers.
Barry Steinhardt, director of the American Civil Liberties Union's technology and privacy program, questions whether the program even works. "I haven't seen any studies on it," he said.
He's particularly concerned behavior pattern recognition might become a pretext for racial profiling.
"Not every police or security officer who's going to be heading up a local operation is going to be sensitive to the racial implications of the project, especially as you roll it out nationwide," Steinhardt said.
The head of the ACLU's racial profiling project aroused suspicions while traveling through Logan in October. He was questioned and lodged a complaint afterward.
Steinhardt claimed "someone thought he was suspicious because he's a tall black man."
Robbins said the incident - the airport's only complaint about racial profiling related to behavior recognition - was being reviewed.
Rafi Ron, former head of security at Israel's Ben Gurion Airport, was hired to train Logan's state troopers in behavior pattern recognition. He said terrorists understand the limitations of security technology. Walk-through metal detectors and X-ray machines for carry-on bags can't detect plastic explosives, for example.
"If you have identified a loophole in the technology's performance, you can rely on the fact that it will always be there and you can use it," Ron said. "To develop such expectations over what will draw suspicion from a law enforcement officer is something that's much less reliable."
Some privacy advocates prefer behavior pattern recognition to the government's plan to conduct computerized background checks on all air travelers to try to identify potential terrorists and other dangerous people.
"Targeted interviewing is certainly preferable," said David Sobel, general counsel for the Electronic Privacy Information Center, a leading critic of the background checks.
Rep. John Mica, R-Fla., chairman of the House aviation subcommittee, said he's been pushing the government to adopt behavior pattern recognition because it helps fill gaps in airport security.
Mica said the current focus on taking dangerous objects from passengers is limited by technology. Too much attention has been focused on keeping items, rather than potentially dangerous people, off planes, he said.
"I don't mind if people carry pocket knives and semi-harmless objects aboard aircraft," Mica said. "What we do mind is if another wave of bad guys gets on an airplane and takes it over."
By Leslie Miller