The commander of international forces in Afghanistan, Gen David Petraeus, has revealed he has spoken to the father of killed aid worker Linda Norgrove.
It was thought her captors had killed her during a rescue attempt but evidence suggests a US grenade may be to blame.
A joint US/UK military inquiry will examine exactly how Ms Norgrove died.
Gen Petraeus said he offered Mr Norgrove his sincere condolences and updated him on the investigation.
Ms Norgrove, 36, was seized in the Dewagal valley in the Kunar province in Afghanistan on 26 September.
Originally from Lewis in the western Isles of Scotland, she was employed by US aid group DAI when she was kidnapped.
Three local staff were taken with her when the two cars they were travelling in were ambushed. The staff were released unharmed but Ms Norgrove was killed during the rescue mission last week.
Gen Petraeus said the new version of events only emerged when footage was removed from a hard drive which showed a "sharper image" of what had happened.
"From that, it was very clear there was a throw-in motion and an explosion followed that and a grenade had been deployed," he said.
The general discussed the issue during a speech at the Royal United Services Institute in London.
He said: "I should note that in fact this morning I talked with Mr Norgrove, and conveyed to him not only the most sincere condolences but also an update on the investigation that is ongoing which is being led by a US 2 star (general) and a UK 1 star (general) - both of whom have extensive experience in special operations."
Gen Petraeus also said Nato had provided safe passage for Taliban leaders to travel to Kabul for face-to-face negotiations with the Afghan government.
He said: "In certain respects we do facilitate that, given that, needless to say, it would not be the easiest of tasks for a senior Taliban commander to enter Afghanistan and make his way to Kabul if ISAF [International Security Assistance Force] were not willing and therefore aware of it and allows it to take place."
He also said more than 300 Taliban leaders had been killed or captured in the past 90 days.
And he said al-Qaeda still had a presence in Afghanistan, although the organisation was under "enormous pressure" with its senior leaders forced to go "very deep underground - figuratively speaking, maybe literally as well".
Gen Petraeus added that the investigation into Ms Norgrove's death was active, would be transparent and its outcome would be shared completely with the Norgrove family.
He said he did not want to speculate on the investigation and added: "It was disturbing, clearly, not to have the correct facts the morning after the operation was conducted and to have those provided later after the task force commander conducted further examination including watching six different videos; six different platforms."
He said "the best operatives in the world" had risked their lives on the mission and also revealed that an AC-130 helicopter gunship, which had been in the area at the time, had killed two of those believed to have been holding her.
Gen Petraeus's visit to the capital was arranged before Miss Norgrove was killed last week during a US special forces rescue attempt sanctioned by Prime Minister David Cameron.
The Foreign Office said her body had been repatriated to the UK on Thursday.
On Thursday, a Downing Street spokesman said: "Gen Petraeus said that the investigation was a personal priority for him and emphasised that there would be full co-operation between the US and UK."
The prime minister's office announced on Wednesday that a British coroner would oversee a post-mortem examination of Ms Norgrove's body.
Gen Petraeus said the next step was for the examination to establish the cause of death.
That inquiry is being led by Brig Rob Nitsch, the Head of Joint Force Support, UK Forces Afghanistan and senior US investigating officer Maj Gen Joseph Votel.