Gunmen ambushed a bus carrying Sunni Muslims and killed all 10 passengers in a troubled northwestern tribal region Saturday, a government official said. Seven people were killed on the spot and three died at a hospital in the Kurram region, said Mohammed Ali, a local government official. At the time of the ambush, the bus was traveling in an area dominated by minority Shiite Muslims, Ali said. He said it was unclear who targeted the bus and officers were still investigating. Kurram has witnessed attacks on Sunni and Shiite Muslims in recent years.