David Cameron: "Linda may not have died at the hands of her captors, as originally believed"
British aid worker Linda Norgrove may have been accidentally killed by US forces during a rescue mission in Afghanistan, David Cameron has said.
International forces there originally said the 36-year-old died on Friday when one of her captors detonated a suicide vest.
But the prime minister said new details had come to light suggesting her death may have resulted from a US grenade.
He said he had spoken to her family about the "deeply distressing" news.
Mr Cameron said he was told of the new developments in a phone call from Gen David Petraeus, the top allied commander in Afghanistan, on Monday morning.
He said the general had told him US forces were deeply dismayed at the outcome and said it was "deeply regrettable" that information published on Saturday about Ms Norgrove was highly likely to have been incorrect.
The BBC's diplomatic correspondent Nicholas Witchell in Kabul said British officials there were "dumbfounded" and he expected them to be "angry at this turn of events".
He added: "It raises questions about the manner of the assault; it raises questions about the way in which the American media operation has disseminated this suggestion that she died at the hands of her captors quite unequivocally for 48 hours."
"It appears there has been a review of the surveillance footage that the Americans have, together with discussions with members of the rescue team, that they cannot 'conclusively determine' - that's their phrase - how Linda Norgrove did in fact meet her death."
He said there had been no suggestion during the past 48 hours the US forces had used grenades - and in this case potential shrapnel grenades - until now.
BBC diplomatic correspondent Bridget Kendall added the latest developments would raise questions over UK and US relations and the possibility there was an attempt to cover up the circumstances of Ms Norgrove's death.
But Foreign Office Minister Jeremy Browne said there had been no attempt to cover up the truth about the rescue operation.
He said: "That was us acting at all stages on the best information that we had supplied to us. But we will have an investigation and we will try and establish as far as is possible, in what sound like completely chaotic circumstances, precisely what happened."
At the Downing Street press conference, Mr Cameron defended the rescue mission, saying it had his full support as Ms Norgrove had been in "grave danger".
He said: "The decision to launch this rescue operation was not an easy one. But I am clear that Linda's life was in grave danger from the moment she was taken.
"Those on the ground and in London feared that she was going to be passed up the terrorist chain which would increase further the already high risk that she would be killed."
Mr Cameron said 12 meetings of the government emergencies committee, Cobra, had taken place before Foreign Secretary William Hague and the US agreed the rescue attempt should go ahead. His decision was then approved by the prime minister.
It had been thought that Ms Norgrove had been killed by her abductors just as US forces reached the compound in which she was being held in Afghanistan.
Linda Norgrove was seized in the province of Kunar on 26 September
But at the start of the press conference, Mr Cameron said it had since emerged that she may have died as a result of a US grenade being detonated during the rescue.
Mr Cameron said it had not yet been confirmed that was the case but a full US/UK investigation - which will last several days - was being launched. The results are expected to be made public.
"We must get to the bottom of what happened, first of all so the family gets this information and knows exactly how their wonderful daughter died," he said.
Mr Cameron told the press conference: "My thoughts and the thoughts of the whole country are with them, as they come to terms with the death of their daughter and this deeply distressing development.
"Linda's death is a tragedy for her family and those who worked alongside her in Afghanistan. She was a dedicated professional doing a job she loved in a country she loved."
Speaking from the Isle of Lewis, Mr Norgrove said: "We are not saying anything to the press at the moment. We might issue a statement in another day or two, we're not certain, but now we are not saying anything."
Mr Cameron added that Ms Norgrove, who was seized in the province of Kunar on 26 September, was being held in remote and high mountains, making the rescue operation very difficult.
Three local staff were also kidnapped alongside Ms Norgrove when the two cars they were travelling in were ambushed. The staff were released unharmed last week.
The Briton, who was employed by US aid group DAI, is believed to have been taken by her captors from village to village as British, Afghan and other intelligence agencies searched the remote area.
The BBC's Bilal Sarwary in Kabul said tribal elders negotiating her release had asked Nato not to intervene, to insure they had more time to secure a release.
And an Afghan intelligence official told the BBC the US had ignored local police and intelligence recommendations supporting this course of action.