An Afghan soldier killed two American soldiers and wounded two more outside a notorious top-security prison near Kabul Sunday.
The gunman was shot dead by other Afghan troops at Pul-e-Charkhi prison, some 30 kilometers (20 miles) east of the capital, said Maj. Sheldon Smith, a spokesman for Combined Security Transition Command, which trains Afghan security forces.
The Americans were working as mentors to Afghan troops providing external security for the prison, Smith said. U.S. and Afghan authorities were trying to determine the motive for the attack, he said.
"All indications are" that the shooter was a member of the Afghan National Army, the post-Taliban force trained mainly by the American military, Smith said.
"The ANA are our partners here and they are working side by side with us in conducting this investigation," Smith said.
He said the U.S. soldiers were in two vehicles when they were shot at. The victims were not identified and Smith provided no further details of the incident.
ANA soldiers with their embedded U.S. trainers have been deployed at the prison in connection with its new high-security wing, Smith said. The wing, which opened last month, is designed to eventually house Afghans released from the U.S. military jail at Guantanamo Bay, Cuba.
The revamp is supposed to improve security at the grim concrete jail, which is infamous among Afghans for tales of torture and appalling conditions dating back to communist rule in the 1970s.
Since the U.S.-led invasion that topped the Taliban, hundreds of al-Qaida and Taliban suspects have been incarcerated there, some of whom have been involved in a series of deadly riots and breakouts.
The jail currently has one American inmate, a former soldier called Jonathan 'Jack' Idema, who was arrested in July 2004 and convicted of running a private prison in Kabul.
Idema is expected to be released soon. Two other Americans arrested along with him were freed last year.