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There were fears Linda Norgrove may have been sold on to an even more ruthless gang
US special forces in Afghanistan were seconds from rescuing a UK hostage when she was almost certainly killed by a suicide bomb vest, the BBC has learned.
They had reached the building where Linda Norgrove was held and were "very, very close" to her, the BBC was told.
It is understood US rather than UK special forces were deployed because they had some knowledge of the terrain.
David Cameron has said it was "right to try" to rescue the 36-year-old aid worker from Lewis, Western Isles.
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The prime minister said: "Decisions on operations to free hostages are always difficult, but where a British life is in such danger, and where we and our allies can act, I believe it is right to try."
BBC correspondent Nicholas Witchell said officials had confirmed Ms Norgrove was killed by an explosion, almost certainly a suicide vest.
He said: "Her captors had become agitated... the only course of action was to mount a rescue mission.
"They were fearful she would be sold on to an even more ruthless group, or taken into Pakistan, where a rescue mission is impossible."
The area where Ms Norgrove was killed was extremely remote
The failed mission, which involved British officials in an advisory role, was authorised by Foreign Secretary William Hague, with Mr Cameron kept informed.
The top US commander in Afghanistan, General David Petraeus, said the troops had done "everything in their power".
He said US troops' thoughts and prayers were with her family during this difficult time.
Ms Norgrove was seized in the eastern province of Kunar on 26 September.
Three local staff were also kidnapped when the two cars they were 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.
Ms Norgrove had been based in Jalalabad, where she supervised US-funded reconstruction programmes in the eastern region of Afghanistan.
'Devastating'
DAI president James Boomgard said the loss of a "beloved friend and respected colleague" was "devastating news".
In a statement, he said: "We are saddened beyond words by the death of a wonderful woman whose sole purpose in Afghanistan was to do good, to help the Afghan people achieve a measure of prosperity and stability in their everyday lives as they set about rebuilding their country.
"Linda loved Afghanistan and cared deeply for its people, and she was deeply committed to her development mission. She was an inspiration to many of us here at DAI and she will be deeply missed."
Ms Norgrove, who had travelled extensively, was an experienced aid worker who had worked in a number of countries.
She worked for the United Nations in Afghanistan and Laos and, prior to that, led a conservation and poverty reduction project in Peru.
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, and has never been under any government control.
DAI carries out aid work, often subcontracted by the United States Agency for International Development.
This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.
There were fears Linda Norgrove may have been sold on to an even more ruthless gang US special forces in Afghanistan were seconds from rescuing a UK hostage when she was almost certainly killed by a suicide bomb vest, the BBC has learned.
They had reached the building where Linda Norgrove was held and were "very, very close" to her, the BBC was told.
It is understood US rather than UK special forces were deployed because they had some knowledge of the terrain.
David Cameron has said it was "right to try" to rescue the 36-year-old aid worker from Lewis, Western Isles.
Related stories
The prime minister said: "Decisions on operations to free hostages are always difficult, but where a British life is in such danger, and where we and our allies can act, I believe it is right to try."
BBC correspondent Nicholas Witchell said officials had confirmed Ms Norgrove was killed by an explosion, almost certainly a suicide vest.
He said: "Her captors had become agitated... the only course of action was to mount a rescue mission.
"They were fearful she would be sold on to an even more ruthless group, or taken into Pakistan, where a rescue mission is impossible."
The area where Ms Norgrove was killed was extremely remote The failed mission, which involved British officials in an advisory role, was authorised by Foreign Secretary William Hague, with Mr Cameron kept informed.
The top US commander in Afghanistan, General David Petraeus, said the troops had done "everything in their power".
He said US troops' thoughts and prayers were with her family during this difficult time.
Ms Norgrove was seized in the eastern province of Kunar on 26 September.
Three local staff were also kidnapped when the two cars they were 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.
Ms Norgrove had been based in Jalalabad, where she supervised US-funded reconstruction programmes in the eastern region of Afghanistan.
'Devastating'
DAI president James Boomgard said the loss of a "beloved friend and respected colleague" was "devastating news".
In a statement, he said: "We are saddened beyond words by the death of a wonderful woman whose sole purpose in Afghanistan was to do good, to help the Afghan people achieve a measure of prosperity and stability in their everyday lives as they set about rebuilding their country.
"Linda loved Afghanistan and cared deeply for its people, and she was deeply committed to her development mission. She was an inspiration to many of us here at DAI and she will be deeply missed."
Ms Norgrove, who had travelled extensively, was an experienced aid worker who had worked in a number of countries.
She worked for the United Nations in Afghanistan and Laos and, prior to that, led a conservation and poverty reduction project in Peru.
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, and has never been under any government control.
DAI carries out aid work, often subcontracted by the United States Agency for International Development.
This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.