As of December 31, 2002, every bag checked onto a U.S. flight must first be run through a bomb detector. More than 1,060 explosive-detection systems and 5,300 trace detectors are currently used for luggage. These systems employ x-rays and computer tomography to scan for suspicious shapes and object densities. But the U.S. Transportation Security Administration is considering alternative devices-including some for passengers-that will identify the chemical signatures of explosives. Two technologies have successfully passed early tests.