Reporting from Charlotte, N.C.— Rick Vetter was rushing to board the Amtrak train in Charlotte, N.C., on a recent Sunday afternoon when a canine officer suddenly blocked the way. Three federal air marshals in bulletproof vests and two officers trained to spot suspicious behavior watched closely as Seiko, a German shepherd, nosed Vetter's trousers for chemical traces of a bomb. Radiation detectors carried by the marshals scanned the 57-year-old lawyer for concealed nuclear materials. When Seiko indicated a scent, his handler, Julian Swaringen, asked Vetter whether he had pets at home in Garner, N.C. Two mutts, Vetter replied. "You can...