KABUL, Afghanistan (AP) — 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 U.S. Embassy, said the bodies of four of the Americans, escorted by FBI personnel, began their journey back to the United States on Wednesday aboard U.S. 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.

Frans said the team was attacked as they made their return trip toward Kabul from their mission to dispense medical care to villagers in remote Nuristan province. They were set upon by gunmen as they got out of their vehicles to take a rest after crossing a swollen river just across the boundary in Badakhshan.

The account squares with that given by the lone survivor — the team's Afghan driver Safiullah — whom Frans said has been released by Afghan authorities after questioning. An official who was familiar with Safiullah's testimony recounted it to The Associated Press. The official requested anonymity because he was not authorized to release details of an ongoing investigation.

According to Safiullah — who goes by one name — an Afghan man in the area offered to help the team as it was trying to cross the river. Two members of the team — including leader Tom Little, an optometrist from Delmar, New York, who had worked in Afghanistan since the late 1970s — rolled up their pants legs and waded in to find a spot shallow enough for the vehicles to ford the river.

After successfully crossing, the team stopped to take a break in a forested area at the side of the road, which ran through a narrow valley. They wanted to get ready for their long journey back though Badakhshan province and on to Kabul.

The Afghan man who had offered to help the group left. Then came the attack. The gunmen rushed in, firing bullets over the medical team members' heads. Moments later 10 of the team were dead.

It appears from accounts of survivors that the gunmen came from the Barg-e-Matal district of Nuristan which is closer to the Pakistani border. Insurgent infiltration has increased in that area since the U.S. abandoned small combat outposts that proved too difficult to defend and resupply.

Safiullah told investigators he believes the lead gunman was Pakistani because he yelled "Jadee! Jadee!" — a word used in several regional languages that means "hurry up." It is more commonly used in Pakistan and India than Afghanistan. He said all the attackers understood Dari and Pashto, the two main languages spoken in Afghanistan, but conversed in Pashaye, a local dialect used only in parts of the northeast corner of Afghanistan.

Safiullah was taken hostage by the attackers and said they walked toward a flashing light that Safiullah said was meant to guide them to a village near Barg-e-Matal, scene of heavy fighting in recent weeks between government forces and militants who crossed over from Pakistan.

The attackers later let Safiullah go and he fled on foot, eventually finding his way back to the town in Nuristan where the group had left their three four-wheel-drive vehicles and rented eight horses at the beginning of the trip.

Frans said that since the killings, International Assistance Mission has received hundreds of e-mails, phone calls and letters. "All but a few have paid tribute to the team members who were killed, to their selfless service and to IAM's commitment to continue working alongside the Afghan people," he said.

In a statement issued Thursday from its headquarters near Geneva, the World Medical Association condemned the attack as a violation of international law.

"It is a tragedy that these doctors were killed while trying to provide medical care to desperate people in need of help. Physicians must always be given free access to patients, to medical facilities and equipment as well as the protection needed to carry out their professional activities," the association said.