International forces in Afghanistan are investigating the circumstances surrounding the death of two Americans, one a soldier, in a checkpoint incident involving Afghan troops.
A US soldier and a civilian contractor were killed in the incident in eastern Afghanistan on Sunday.
The coalition initially said the incident may have been the result of an Afghan soldier turning his gun on the Americans but later said that nearby insurgent gunfire could have led to a misunderstanding.
Lieutenant General Adrian Bradshaw, deputy commander of the NATO-led coalition, said the circumstances of the shooting were "somewhat confused".
The soldier's death takes the toll of American military deaths in the Afghan war to 2000.
It was the latest setback for the coalition after the United States said joint operations with Afghan forces were returning to normal after a surge of attacks on NATO-led soldiers by their Afghan allies.
Joint operations were halted two weeks ago after a increase of attacks on the International Security Assistance Force (ISAF) by its Afghan allies.
At least 52 ISAF service members have been killed this year in so-called "green-on-blue" attacks.