As the man approached the airport security checkpoint here on Wednesday, he kept picking up and putting down his backpack, touching his fingers to his chin, rubbing some object in his hands and finally reaching for his pack of cigarettes, even though smoking was not allowed.

Two Transportation Security Administration officers stood nearby, nearly motionless and silent, gazing straight at him. Then, with a nod, they moved in, chatting briefly with the man, and then swiftly pulled him aside for an intense search.

Another airline passenger had just made the acquaintance of the transportation agency's "behavior detection officers."