The Transportation Security Administration plans to train screeners at 40 major airports next year to pick out possible terrorists by engaging travelers in a casual conversation to detect whether a person appears nervous or evasive and needs extra scrutiny.

The new security technique, already in use at some airports, adds a psychological dimension to screening by trying to find high-risk passengers based on how they act at checkpoints or boarding gates.

Passengers who raise suspicions will undergo extra physical screening and could face police questioning.

Airports in New York, Los Angeles, Chicago, Houston, Detroit and Miami recently began using the technique.

Some airport and transit police already look for people acting oddly -- such as wearing a heavy coat in the summer or appearing to be doing surveillance -- and question them about travel plans.

"I don't want (officers) just sitting there waiting for a call to come in. I want them observing people, observing their behavior and engaging them in conversation. They're looking for people whose activities don't look right," says Alvy Dodson, public safety director at Dallas/Fort Worth International Airport. Last year, 70% of DFW's 167 airport police were trained in the program.