KABUL, Afghanistan — A grenade that may have killed a British aid worker during an attempt to rescue her from the Taliban was thrown by a U.S. Navy SEAL, a senior defense official told NBC News Thursday.

The source was not certain whether all of the U.S. forces involved in conducting the rescue operation were SEALS, but added, "they usually work in one team made up of only SEALS."

NATO had initially said Linda Norgrove, 36, was killed by a Taliban bomb during the operation on Friday last week.

However, the coalition said Monday that a review of surveillance tapes suggested it was possible she may have been killed mistakenly by people trying to rescue her.

The U.K.'s Guardian newspaper, citing sources in Kabul and London, reported Thursday that Norgrove had managed to break away from her captors in the darkness of a moonless night.

Fetal position
She then lay down in the fetal position, apparently to try to avoid being hit during the fighting, the paper said.

It added that the cause of death was still to be determined by an autopsy, but that Coalition commanders believed her fatal injuries were caused by the grenade.

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On Tuesday, President Barack Obama promised to "get to the bottom" of the failed rescue effort.

Norgrove, originally from Sutherland in the Scottish Highlands, worked on a U.S.-funded aid project for Development Alternatives Inc., a Bethesda, Maryland-based organization.

She was abducted in an ambush on Sept. 26 while driving toward Asadabad, the capital of Kunar province, according to Afghan officials. She was to oversee projects in the area.

Norgrove had worked in Afghanistan for years on various aid projects, spoke the language and was "dedicated to Afghanistan," according to a statement released by her employer. Her projects mainly involved working with farmers or on environmental protection programs.

She had donned a burqa — a body-covering robe worn by many Afghan women — for the trip during which she was kidnapped, local police said.

Three Afghan colleagues were also captured in the ambush but all were later released.

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When the rescue team assaulted the Taliban hideout, they came under fire from within the compound as well as from an over-watch position nearby, said Lt. Col. John Dorrian, a spokesman at NATO headquarters in Kabul on Monday.

All six gunmen who fought back against the U.S. force were killed, along with Norgrove. He said women and children in the compound were not hurt, and no one on the U.S. rescue team was wounded.

U.K. Prime Minister David Cameron has defended the rescue mission, saying it was clear that Norgrove was "in grave danger and the operation offered the best chance of saving her life."