The death of Karen Woo – by the man who survived

They were resting in the shade of trees, exhausted but relieved that they had managed to get their vehicles through a swollen river when the attackers appeared. From nowhere, men began shouting orders and firing over the medical team's heads.

Tom Little, a doctor who had dedicated 40 years of his life to helping the Afghans, called out "What's happening?" before a blow to the head with an AK-47 rifle sent him reeling to the ground. As he struggled to get up, he was fatally shot in the chest. One by one, the nine other members of his team were executed as they tried in vain to shield themselves from bullets and grenades.

For the first time, the lone survivor of the killing of 10 members of the International Assistance Mission (IAM) – which included the British doctor Karen Woo – has described the horror of their last moments.

Dr Woo, a "vibrant, energetic and dedicated" 36-year-old who was due to marry this week, had ignored pleas from friends and family, determined that the essential nature of the work outweighed the perils. Her body and those of six Americans, two Afghans, and one German, were found in the Koran Wa Munjan district of Badakhshan on 6 August.

The driver, called Safiullah, revealed that it was an act of kindness – a lift to strangers – that would lead to the murders. At his home in a district of Kabul, the 28-year-old spoke hesitantly as he recounted the events that have left him traumatised, in fear of his life and unable to sleep. While suspicion initially fell on him, he has now been released by the Afghan authorities after questioning.

Frequently breaking down in tears, the father of four paused constantly as he remembered his colleagues. "God was good to me," he explained simply.

The IAM team had spent the previous two weeks covering about 100 miles – much of it on foot and horseback – through the Hindu Kush mountains, bringing medical care to impoverished villagers in Nuristan province. The day before they were killed, the team was guided back by locals to their vehicles, so they could start the final leg of their trip home.

They soon stopped to give three men a lift, a charitable courtesy in the rugged and remote area. Two of the men set off on their way when the vehicles were blocked by a river, while the third "quickly disappeared", Safiullah explained.

Dr Little, the team leader, and another member waded into the waters to find a shallow place for the vehicles to cross safely. A short while later, they were resting in a forested area in the narrow valley, preparing themselves for the long trip back through Badakhshan province and on to the Afghan capital.

Suddenly ten gunmen shouting "satellite, satellite" – a demand to surrender their phones – appeared and the driver recognised the third pedestrian from earlier. They seemed to be motivated, skilled and organised militants, some wearing commando-style gear, he said.

After killing Dr Little, the gunmen turned their attention to two of the three female members of the team who were hiding inside one of the vehicles and threw a grenade at it, killing them both. Then they shot the team's Afghan cook, who had used luggage to barricade himself under the car. As the gunmen murdered the rest of the group one by one, the driver begged for his life, raising his arms in the air as he recited verses from the Koran. Safiullah believed their commander – a "tyrant with a cruel face" – was Pakistani because he yelled "Jaldee! Jaldee!", or hurry up, a term more common over the border than in Afghanistan. But the rest of the gunmen seemed to come from Nuristan province because they conversed in the local dialect Pashaye.

Asked why the gunman did not spare his fellow Afghans, the driver speculated the cook might have been targeted because they thought he had a satellite phone and the second man, a guard, had been wearing a head scarf similar to a bodyguard.

"They had made a plan," Safiullah recalled. "It was a very organised group. They had leadership. They were well-organised. They were militants. If it's 100 years later and I see them, I'll know them."

The killers took his wedding ring and $50 from his pocket before loading him down with weapons and luggage for an eight-hour walk. On a radio, one of the gunmen reported back: "Everything's finished. We killed them."

After being met by another group, the driver was interrogated about his faith, family and why he worked for foreigners before a gunman kicked him to the ground and told him he was free.

Fearful they would hunt him down and kill him, he initially clung to one of the gunmen's legs before eventually running for his life. Exhausted and having not eaten for two days, he was resting by a large rock when an older shepherd offered to take him back to his house in the Naw village, where the authorities found him.

Back at the scene of the murders, the young driver helped police load the bodies of his former colleagues into the two remaining four-wheeled drive vehicles to start their journey back to Kabul.

"Psychologically, I am not well," Saifullah admitted. "My concern is about my life. I'm not feeling safe." Lowering his eyes, he continued: "In the history of my life. I will never forget this."

He said he did not know whether he could go back to helping Western aid agencies: his attackers had warned him not to have anything to do with foreigners or the Afghan authorities.

While the killings were initially blamed on a robbery, the Taliban claimed the credit for them, insisting the workers were trying to convert people to Christianity, a statement IAM strenuously denied. Dirk Frans, executive director of the NGO, has since said IAM believes its workers were victims of "an opportunistic ambush by a group of non-local fighters".

The murders have been described as the worst attack on aid workers in 30 years and a shattering set back for those attempting to bring aid to the country. They came at a time when civilian deaths have jumped 31 per cent in a year and the number of Nato troops killed has passed the 2,000 mark.

The senior military commander in Afghanistan, General David Petraeus, may have last night added to the confusion over Western policy in Afghanistan by saying it was still unclear whether President Barack Obama's deadline to start pulling out troops next year could be met.

In a television interview designed to bolster support after a recent poll found that seven out of 10 Americans did not believe the war would end successfully, General Petraeus said he had seen "areas of progress" but battling the Taliban was an "up and down process".

He said he would give his "best professional military advice" to Mr Obama about the July 2011 deadline, adding: "I think the President has been quite clear in explaining that it's a process, not an event, and that it's conditions-based. It would be premature to have any kind of assessment at this juncture about what we may or may not be able to transition."