Federal law enforcement authorities are investigating a nascent plot to carry out a series of terrorist bombings at stations in the Washington Metro system, according to intelligence and law enforcement sources. The investigation is focused on a naturalized U.S. citizen, originally from Pakistan, who became the target of an undercover sting operation, the sources said. An administration official said the man drew the attention of law enforcement officials by seeking to obtain unspecified materials. The planned attack was not imminent, the sources said, speaking on condition of anonymity because the matter remains under investigation.