Kabul, Afghanistan ? An Afghan soldier shot and killed two U.S. soldiers and wounded two more Sunday outside a top-security prison near Kabul, a U.S. military spokesman said.

The gunman was shot dead by other Afghan troops at Pul-e-Charkhi prison, some 20 miles east of Kabul, said Maj. Sheldon Smith, a spokesman for Combined Security Transition Command, a body that trains and mentors Afghan security forces.

The Americans were providing external security for the prison when they were shot, Smith said. U.S. and Afghan authorities opened investigations into the shooting and were trying to determine the motive for the attack, he said.