The agency representing eight slain medical workers in Afghanistan says that it now believes the attacks were committed by insurgents rather than robbers.

"Our own research suggests that the murders were not a robbery as initially reported in the press. We are now working on the assumption that the attack was an opportunistic ambush by a group of non-local fighters," said Dirk Frans, executive director of International Assistance Mission (IAM), a Christian relief agency.

According to Frans, the group of six Americans, two Afghans, on German, and one Briton, were attacked last Thursday while resting after a difficult crossing of a swollen river.

"After making it safely to the other side of the river they got out of their vehicles obviously relieved at the successful crossing and end of the difficult part of their trip. It was then that a group of armed men attacked the team killing all but one of its members on the spot," Frans said.

Frans' report lines up with a recounting of the incident from Safiullah, the group's driver and the lone survivor of the attack, whose eyewitness testimony was shared with the Associated Press.

According to Safiullah, the gunmen were likely Pakistani based on the language that they spoke and were probably from a district in northeast Afghanistan near the Pakistani border.

Safiullah was released by the gunmen after begging for his life and reciting verses from the Koran to prove he was Muslim.

Last week's attack, which is considered the most severe on NGO workers in years, has been a shock to the international community, which has issued calls for increased protection of aid workers in the region.

A search for the killers is still ongoing, although the Taliban has publicly declared responsibility for the attack, claiming that the workers were proselytizing.

The loss has temporarily impaired IAM's ability to work in the region but Frans said the group has no intentions to leave.

On Thursday, persecution watch-dog group Release International issued a call to prayer for protection and peace over believers living and working in Afghanistan.

"The Christian community have a vital part to play in the building of the nation, inspired by God to serve, driven with passion to love, and selflessly focused on those more needy than themselves," said Release CEO Andy Dipper.