Americans killed in explosion at U.S. base in Afghanistan

KABUL, Afghanistan -- An explosion at a U.S. airfield in Afghanistan early Saturday killed Americans, Defense Secretary Ash Carter said.Carter said in a statement that two U.S. service members and two U.S. contractors working at Bagram Air Field were killed by “an apparent suicide bomber.”The explosion wounded 16 other U.S. service members and one Polish soldier at the base, Carter said.The incident was being investigated, he said.“For those who carried out this attack, my mess