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.

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