KABUL, Afghanistan — The U.S. military launched a high-level investigation Monday to determine whether a captured British aid worker was killed accidentally by the American rescue team, not by her Afghan captors, as officials originally said.

Forty-eight hours after they blamed Taliban kidnappers for killing Linda Norgrove, American officials said that a review of the incident indicated that the 36-year-old worker may have been mortally wounded when U.S. forces threw a grenade into the room where she was held.

British Prime Minister David Cameron announced the sobering news in London after U.S. Army Gen. David Petraeus, the commander of the international military coalition in Afghanistan, informed him of what Cameron called the “”deeply distressing development.””

The revelation could increase opposition to the Afghan war in the United Kingdom, where support for the U.S.-led campaign to shore up the pro-Western Kabul government and prevent Taliban forces from returning to power already is flagging.

It also raised questions about why military officials in Afghanistan initially blamed the Taliban for killing Norgrove with a suicide belt.

Militants abducted Norgrove and three Afghan colleagues on Sept. 26 as they drove in two unarmored cars through Kunar province, which borders Pakistan. While the Afghan captives were freed last week, the militants held Norgrove at a compound in eastern Afghanistan’s mountains.

Addressing the British parliament Monday, Foreign Secretary William Hague said that Norgrove was being held by militant Islamic insurgents aligned with the Taliban who had no intention of freeing her.

“”Nothing that happened between 26th of September and 8th October caused me or anyone else involved to change our view that a rescue operation was the only realistic hope for Linda’s safe and secure release,”” Hague said.

Over the weekend, in consultation with British officials, the U.S. military ordered the rescue mission after concluding that Norgrove was in danger of being spirited into Pakistan and turned over to even more recalcitrant insurgents.

Initially, military officials in Afghanistan said that a Taliban captor used an explosive device — most likely a suicide belt — to kill Norgrove and himself as the rescue team closed in. But questions about that story began to arise during a military review of surveillance video and the debriefing of the rescue team.

“”Evidence, and subsequent interviews with the personnel involved, suggests that Linda could have died as a result of a grenade detonated by the task force during the assault,”” Cameron said in his televised news conference.

Petraeus called for a special investigation, which will be led by Army Maj. Gen. Joseph Votel, the deputy head of U.S. special forces.

Some military officials in Kabul appeared to have been blindsided by the news and were at a loss to explain why Western officials had been so quick to point the finger at Norgrove’s captors.

“”There’s a well-worn term in military channels and it’s that first reports are always wrong,”” a U.S. military official said. “”I think maybe we saw an indication of that here.””

A Western official who’d seen photographs of the scene said that one Afghan captor was found dead alongside Norgrove, who was fatally wounded. That, he said, may have led to the presumption that the captor caused the explosion that killed Norgrove.

Both officials agreed to discuss the incident only on the condition of anonymity, because they weren’t authorized to talk to journalists about its details.

Despite the news, Cameron defended the decision to attempt a rescue.

“”The best chance of saving Linda’s life was to go ahead, recognizing that any operation was fraught with risk for all those involved, and success could by no means be guaranteed,”” he said.

At the time of her abduction, Norgrove was working for Development Alternatives Inc., an international consulting firm based in Washington.

Norgrove had worked in Afghanistan off and on since 2005. She was spearheading part of a $150 million, five-year U.S. Agency for International Development project.