Kabul, Afghanistan ? An examination of video footage documenting a failed attempt to rescue a kidnapped British aid worker shows that she may have been accidentally killed by a grenade thrown by U.S. Special Operations troops involved in the raid, according to U.S. and British officials.

The new account of the death of Linda Norgrove, a 36-year-old British woman taken hostage in eastern Afghanistan last month, contradicted the initial assertion by officials in Kabul that one of her captors had detonated a bomb as the rescue team was closing in.

At a news conference in London after receiving a call from Gen. David Petraeus, the top American commander in Afghanistan, British Prime Minister David Cameron disclosed the possibility that U.S. forces had accidentally killed Norgrove.

The disclosure raised questions about whether the U.S. Special Operations forces made mistakes during the pre-dawn raid Friday, and it left unanswered why the military had so definitively asserted on Friday that Norgrove was killed by a Taliban bomb.

Several U.S. military officials in Afghanistan said they believed the new information only came to light Monday morning after commanders sorted through the details of a chaotic rescue attempt that left six insurgents dead, as well as Norgrove.

A commander of the Special Operations team, who watched video footage of the raid taken from an overhead surveillance drone, discovered what “looked to him like someone throwing a hand grenade into the area where Miss Norgrove was being held,” said Navy Capt. Gary Kirchner, a NATO spokesman in Kabul.

“You’ve got a messy situation and it takes time to figure it out,” said a U.S. military official in Kabul. “We thought the situation was something, and we realized it’s something else.”

The possibility that American forces killed Norgrove is troubling news for the British government, which is already confronting souring public opinion on the war and increasing pressure to pull out its troops, who make up the second-largest foreign contingent in Afghanistan after the United States. The incident has the potential to stiffen that opposition, particularly if officials are seen as having acted late in contradicting accounts that initially put the blame for the worker’s death on her captors.

In light of the new account, Petraeus ordered an investigation of Norgrove’s death, to be conducted by U.S. Central Command in Tampa, Fla.