KABUL, Afghanistan — A Christian charity group said Thursday that it believes militants, not robbers, killed 10 members of its medical team last week in a remote area of northern Afghanistan.In the first days after the attack, the group's leaders had said they suspected the team was set upon by robbers, despite a Taliban claim of responsibility. Local police had also said they suspected a criminal motive behind the killing of six Americans, two Afghans, a German and a Briton in the northern province of Badakhshan."Our own research suggests that the murders were not a robbery," Dirk Frans, director of the International Assistance Mission, said in a statement. "We are now working on the assumption that the attack was an opportunistic ambush by a group of non-local fighters."Caitlin Hayden, a spokeswoman for the US Embassy, said the bodies of four of the Americans, escorted by FBI personnel, began their journey back to the United States on Wednesday aboard US military aircraft. They were flown from Kabul to Bagram Air Field, outside the capital, and from there, they will continue on to the United States.