KABUL - British Prime Minister David Cameron said Monday morning that Linda Norgrove, a 36-year-old British aid worker who was taken captive in Afghanistan last month, may have been accidentally killed during a rescue attempt by U.S. Special Operations Forces.

Norgrove's death was blamed on one of her captors, who officials said had apparently detonated a bomb as U.S. and Afghan troops were closing in. But Cameron said Monday that new details indicated she may have been killed instead by a grenade used by the Americans during the rescue mission.

Cameron said he received a call informing him of fresh information in the case early Monday from Gen. David H. Petraeus, the commander of U.S. and NATO forces in Afghanistan.

"We must get to the bottom of what happened, first of all so the family gets this information and knows exactly how their wonderful daughter died," Cameron said at a news conference at No. 10 Downing Street. He said he had already spoken with Norgrove's parents to keep them apprised.

Cameron strongly defended the mission itself, saying it had been agreed upon after intense consideration and exchanges between British and American officials. He said a green light was given because Norgrove was in serious danger, being held by Taliban fighters in a remote mountain valley of Konar province in eastern Afghanistan.

"The decision to launch this rescue operation was not an easy one," Cameron said. "But I am clear that Linda's life was in grave danger from the moment she was taken."

"Those on the ground and in London feared that she was going to be passed up the terrorist chain, which would increase further the already high risk that she would be killed," he said.

Immediately after her death, NATO officials said one of her captors killed her by detonating an explosive as the Special Operations Forces rescue team was approaching for its pre-dawn raid. But after receiving new information from the Special Operations Command that conducted the rescue attempt, Petraeus ordered an investigation into the circumstances of her death.

"Subsequent review of surveillance footage and discussions with members of the rescue team do not conclusively determine the cause of her death," a NATO statement said.

Norgrove, who worked for DAI, a Bethesda-based contractor for the U.S. Agency for International Development, had spent several years in Afghanistan, spoke fluent Dari and was learning Pashto, the two most commonly spoken languages here. Based in the eastern city of Jalalabad, she was traveling to Konar with three Afghan colleagues as part of their work on an irrigation project. Her colleagues escaped unharmed, officials said.

Navy Capt. Gary Kirchner, a NATO spokesman in Kabul, said the commander of the Special Operations troops involved in the rescue attempt reviewed video footage from a helmet camera on Monday morning.

The commander "discovered what appeared to be, looked to him, like someone throwing a hand grenade into the area where Miss Norgrove was being held," Kirchner said.

The commander then called Petraeus, prompting the investigation.

"The bottom line here is that when the commander saw that, he knew that there were some discrepancies and immediately wanted to make sure that we did the right thing," Kirchner said.

He said that Central Command in Tampa will be organizing the investigation, which will involve reviewing the mission plan, interviewing the Special Operations Forces team members and examining video footage and communications that took place during the attempted rescue.

"This is critically important, especially for the family," he said.

Faiola reported from Sweden.