An investigation was launched today into the death of a British aid worker in Afghanistan after it emerged that she may have been killed by a grenade thrown by US special forces trying to rescue her.
US officials initially said Linda Norgrove, 36, died after her captors detonated a bomb vest as American troops attempted to free her from militant kidnappers on Friday night.
But General David Petraeus, the American commander of the Nato-led Isaf force in Afghanistan, told Prime Minister David Cameron today that a second viewing of video recordings of the incident suggested that Ms Norgrove might have been the victim of "friendly fire".
In a statement, the US military said that a review of surveillance footage and interviews with members of the rescue team "do not conclusively determine the cause of her death".
The investigation was initiated by US Central Command and members of UK forces will be invited to contribute, said a spokesman.
Describing Ms Norgrove's death as "a tragedy", Mr Cameron promised to do "everything I possibly can" to give her family certainty about how she died.
The Prime Minister personally informed her family about the "deeply distressing development" in a phone call to her father John which delayed a scheduled Downing Street press conference this morning by almost an hour.
He told reporters the initial reports of how the hostage died were "deeply regrettable" but were made in good faith on the basis of the information available.
Mr Cameron praised the "bravery" shown by American troops, and said it would have been "quite unorthodox" for him to overrule commanders on the ground and insist on British special forces undertaking the rescue attempt in an area controlled by US forces.
"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," said the Prime Minister.
"I am clear that the best chance of saving Linda's life was to go ahead, recognising that any operation was fraught with risk for all those involved and success was by no means guaranteed."
Speaking from the Isle of Lewis after his conversation with the Prime Minister, 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."
The aid worker's former headteacher Ewan Mackinnon told the BBC that the possibility of her having been killed by her rescuers would be "even more tragic and even more heartbreaking for her parents".
Originally from Sutherland in the Scottish Highlands, Ms Norgrove was working for the US firm Development Alternatives Inc (DAI) in the east of Afghanistan when she was seized by militants in Kunar province on September 26.
Mr Cameron said that from the moment of her capture, it was recognised she was in "grave danger", with fears that she might be passed up the terrorist chain to Taliban strongholds in Pakistan.
Foreign Secretary William Hague decided early on - with Mr Cameron's "full blessing" - to give the go-ahead for a rescue attempt, and the decision was kept under review in 12 meetings of the Cobra emergency committee over the past fortnight.
In a statement to the House of Commons today, Mr Hague said that British special forces liaised with their American counterparts, but did not play any role in planning or carrying out the rescue attempt.
Flanked by Mr Cameron, the Foreign Secretary told MPs: "All such rescue operations involve a measure of risk which has to be weighed against a constant risk to a hostage and a risk that such an opportunity to undertake a rescue operation may not recur.
"Our objective throughout was clear: to secure Linda Norgrove's safe release while continuing the long-standing policy of successive British governments not to make concessions to hostage takers."
The Foreign Secretary said US special forces had been placed on a 30-minute standby to launch a rescue attempt as soon as Ms Norgrove was located. The group holding her was known to have links with al Qaida, the Taliban and other terrorist groups operating on the Afghan-Pakistan border, he said.
He paid tribute to the US forces "who risked their own lives to try to rescue a British citizen", adding: "We should also remember that the responsibility for the loss of Linda's life lies with those who took her hostage."
Gen Petraeus, who will visit London for scheduled talks with Mr Cameron on Thursday, said it was "our solemn responsibility to understand the circumstances which led to her death".
He added: "We will provide every measure of support to the investigation and will work closely with the British Government to fully resolve this matter."
Nato secretary general Anders Fogh Rasmussen said the alliance would await the outcome of the investigation before commenting.
But he added: "Whatever happened, I would like to stress that those who are responsible of course are the captors."
Shadow foreign secretary Yvette Cooper condemned the actions of the hostage takers, but told MPs there was "concern" about the "potentially inaccurate information that was disseminated over the course of the weekend".
Foreign Office minister Jeremy Browne insisted there had been no attempt to cover up information about how Ms Norgrove died.
He said: "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.
"We are determined to operate completely openly. We are not trying to hide anything. There is no certainty at this stage about what happened."