KABUL, Afghanistan — NATO will investigate whether a grenade thrown by American military forces killed a British aid worker during a rescue attempt in Afghanistan last week, an alliance spokesman said today.

Linda Norgrove, 36, was killed Friday in the raid by U.S. forces in Afghanistan's eastern Kunar province, after she and three colleagues were kidnapped two weeks earlier. NATO initially said Norgrove died when captors detonated a bomb as NATO forces attempted to free her.

However, British Prime Minister David Cameron said today that Norgrove was possibly killed by a grenade lobbed by a member of the U.S. Special Forces rescue team.

Cameron said he had informed Norgrove's family of the "deeply distressing development," and defended the decision to attempt the risky rescue mission.

"We were clear that Linda's life was in grave danger and the operation offered the best chance of saving her life," Cameron told reporters in London.