KABUL, Afghanistan (AP) -- At first, NATO blamed a Taliban bomb for the death of a captive British aid worker during an American rescue attempt in eastern Afghanistan.
Two days later, the coalition changed its account, saying Monday that U.S. forces may have detonated a grenade that killed Linda Norgrove during the operation to free her.
British Prime Minister David Cameron defended Friday's rescue mission, saying his government authorized it only after learning that Norgrove's life was in grave danger. The U.S. military, which carried out the raid because the aid worker was being held in a region under American command, said it would investigate the incident with British cooperation.
In Brussels, NATO Secretary-General Anders Fogh Rasmussen emphasized that "whatever happened, I would like to stress that those who are responsible of course are the captors."
The U.S.-led NATO force has historically been slow to acknowledge friendly-fire deaths in Afghanistan. Drawn-out investigations mean findings can come weeks or months after an incident. But an increased focus on preventing civilian deaths has led NATO over the past year to push for quicker reporting on mistakes.
Norgrove, 36, from Scotland's Isle of Lewis, worked on a U.S.-funded aid project for Development Alternatives Inc., a Bethesda, Maryland-based organization. She was abducted in an ambush on Sept. 26 while driving toward Asadabad, the capital of Kunar province, according to Afghan officials. She was to oversee projects in the area.
Three Afghan colleagues were also captured in the ambush but all were later released.
Norgrove died Friday night - nearly two weeks after being captured - when U.S. special forces stormed the Taliban compound where she was being held in Kunar province.
In its initial statement Saturday, NATO said Norgrove was killed when captors detonated a bomb during the attack.
But then the rescue mission leader saw surveillance footage of the incident, had discussions with other team members and decided "it was not conclusive what the cause of her death was," said Lt. Col. John Dorrian, a spokesman at NATO headquarters in Kabul.
When the rescue team assaulted the Taliban hideout, they came under fire from within the compound as well as from an overwatch position nearby, Dorrian said Monday.
"It was a very high elevation area, very very challenging terrain," Dorrian said. All six gunmen who fought back against the U.S. force were killed, along with Norgrove. He said women and children in the compound were not injured and that no one on the U.S. rescue team was wounded
Dorrian did not provide details on how long the fighting lasted, the size of the force or what weaponry they used.
In London, Cameron said that the U.S. and NATO commander in Afghanistan, Gen. David Petraeus, informed him that Norgrove was possibly killed by a grenade detonated by a member of the rescue team.
Cameron said he told Norgrove's family of the "deeply distressing development," and defended the decision to attempt the risky rescue mission.
"We were clear that Linda's life was in grave danger and the operation offered the best chance of saving her life," Cameron told reporters at a news conference at 10 Downing St.
"I want to assure Mr. and Mrs. Norgrove that I will do everything I possibly can to establish the full facts and give them certainty about how their daughter died," he added. Norgrove's father, John, said the family had no comment.
Cameron said Norgrove's family had been kept informed of the decision, which was made by Foreign Secretary William Hague with his full support.
Hague, in a statement before the House of Commons, said Norgrove's captors intended to "pass her further up the Taliban command chain."
He said intensive efforts to locate Norgrove began immediately after her abduction, including increased military operations in the area where she was taken and leaflet drops offering a reward for information about her whereabouts.
Hague said he had authorized a rescue operation from day one but bad weather prevented an earlier rescue attempt
Norgrove had worked in Afghanistan for years on various aid projects, spoke the language and was "dedicated to Afghanistan," according to a statement released by her employer. Her projects mainly involved working with farmers or on environmental protection programs.
She had donned a burqa - a body-covering robe worn by many Afghan women - for the trip during which she was kidnapped, local police said.
It wasn't the first time in Afghanistan that an operation to rescue an abducted Briton ended in bloodshed. In September 2009, Stephen Farrell, a reporter for The New York Times, and Sultan Munadi, an Afghan journalist and interpreter who worked regularly with the Times and other news organizations, were taken hostage when they went to cover the aftermath of a NATO airstrike that killed scores of civilians in northern Afghanistan.
Munadi and a British commando died in the raid that rescued Farrell, a Briton. British forces said they had to leave Munadi's body behind because they were coming under such heavy fire.