The Bush administration raised the terror alert a notch to code orange for the nation's mass transit systems on Thursday, responding to a spate of deadly rush-hour bus and subway bombings in London. "Obviously we're concerned about the possibility of a copycat attack," said Homeland Security Secretary Michael Chertoff. The heightened alert will apply to "regional and inter-city passenger rail, subways and metropolitan bus systems," Chertoff said at a news conference. Chertoff said that U.S. authorities have "no specific credible evidence" pointing toward an attack in the United States. At the same time, he said, "we are also asking for increased vigilance" particularly in the U.S. transportation system.