Petraeus orders probe into failed UK aid worker rescue
The US commander in Afghanistan, Gen David Petraeus, has ordered an investigation into the death of a British aid worker held hostage.
Linda Norgrove's death on Friday as US forces tried to rescue her was initially blamed on her Afghan captors.
But Prime Minister David Cameron said she may have been accidentally killed by a US grenade.
US military sources said surveillance of the operation from overhead and on the ground was conflicting.
The joint US-UK investigation into the failed rescue attempt will be led by US Maj Gen Joseph Votel, the chief of staff of the US Special Operations Command, the sources told the BBC's Washington correspondent, Adam Brookes.
The investigators will look at surveillance footage of the operation taken by helicopters or pilotless drone aircraft as well as footage from cameras mounted on the helmets of the soldiers on the ground, the sources said.
The different angles reveal "conflicting evidence" as to whether Ms Norgrove was killed by a US grenade, an Afghan suicide vest, or both, they said.
The investigators will also interview the US soldiers who took part in the operation and possibly attempt to return to the site, in a remote and mountainous area of north-eastern Afghanistan near the border with Pakistan.
BBC correspondents say Ms Norgrove was being held by a militant group based in Kunar, some of whose members are closer to al-Qaeda in ideology than to mainstream Taliban.
Six of them were killed, none were detained and none escaped, the military sources said. Sources also said there had been no casualties among the Special Forces team that tried to save Ms Norgrove.
It had been thought that she was killed by her abductors just as US forces reached the compound in which she was being held.
But Mr Cameron said Gen Petraeus had telephoned him on Monday morning to say she could have died as a result of a grenade detonated by the taskforce during the assault.
Ms Norgrove, 36, from the Isle of Lewis in Scotland, was employed by US aid group DAI. She was seized in the Dewagal valley in Kunar province on 26 September.
Three local staff were kidnapped with her when the two cars they were travelling in were ambushed. The staff were released unharmed last week.
The BBC's Bilal Sarwary in Kabul said the Dewagal valley, in eastern Kunar province, where she was held, is known for its difficult terrain. It is mountainous and densely forested. The valley is extremely remote.
The investigation is expected to take several days and the findings released to the public after Ms Norgrove's family has been informed.
At a Downing Street press conference on Monday, 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. This decision was then approved by the prime minister.
He said it was feared that Ms Norgrove life was in danger from the moment she was kidnapped and that she "was going to be passed up the terrorist chain which would increase further the already high risk that she would be killed."
The prime minister's office said Mr Cameron spoke to US President Barack Obama late on Monday and both agreed the decision to launch the rescue attempt was right.
"The prime minister and the president agreed that it was now essential to get to the bottom of what had happened in the course of the rescue operation," a spokesperson for Mr Cameron said.
US officials told the BBC no ransom demands had been received from the kidnappers.
The BBC's Kabul correspondent said tribal elders negotiating her release had asked Nato not to intervene, to ensure they had more time to secure her freedom.
An officer working for the National Directorate of Security, Afghanistan's spy agency, said a delegation of mullahs, tribal elders and village chiefs was despatched to the area soon after her capture to negotiate with the militants.
But the coalition forces bombed several nearby locations, forcing the delegation to halt their mission, our correspondent said.