Officials spoke on condition of anonymity Wednesday because details of the incident were sketchy and the officials weren't authorized to talk about it publicly.
Early reports from Afghans said a car packed with explosives rammed into vehicles in an international military convoy. But U.S. Defense Department officials say it's not clear whether it was a convoy. A suicide vest is suspected to have been used.
While the Americans believed killed were all members of the military, U.S. officials say the Afghans appear to be a mix of security forces and civilians.
