MEXICO CITY (Reuters) - Three U.S. citizens were among those killed when gunmen attacked buses in the eastern Mexican state of Veracruz, said a U.S. State Department official said on Saturday. The three were traveling for the holidays when they and several other passengers on the bus were killed by gunmen on Thursday, according to the U.S. official and local media. The incident was one of several that day in which gunmen attacked busses in the eastern state, a major oil export hub that has lately become a flashpoint for drug gang violence. On Friday, the tortured bodies of 10...