Gunmen fired at a U.S. military-chartered helicopter delivering supplies to troops on a southern Philippine island where Muslim militants have a presence, slightly wounding an American civilian, an embassy official said Tuesday. The helicopter carrying American civilians flew back to a Zamboanga city airfield without any difficulty after it was hit by small-arms fire Monday over nearby Basilan island, U.S. Embassy deputy Press Attache Karen Schinnerer said. An American aboard was slightly injured by flying debris, Schinnerer told The Associated Press without elaborating. Marine Brig. Gen. Juancho Sabban, a Filipino commander, said the helicopter had been on a routine logistical run and was flying back to Zamboanga city when it came briefly under fire near Tipo Tipo township in Basilan, about 850 kilometers (530 miles) south of Manila. The U.S. military was investigating the attack, Schinnerer said, adding that it had not prompted any changes in U.S. military ground or air activities in the southern Mindanao region.