WASHINGTON -- Authorities in Thailand, acting on information from American investigators, seized a large amount of radioactive material from a Thai man, breaking up a plot to sell it to terrorists. He was peddling it for use in so-called "dirty bombs," according to American law enforcement officials. American officials said the seized material -- which Thai authorities said was cesium-137, a radioactive byproduct of nuclear power plants commonly found in medical equipment -- was believed to have originated in Russian stockpiles and been taken to Thailand through Laos. It could easily have been used in terrorist weapons, the officials said....