LONDON—A grenade thrown by U.S. Special Forces killed kidnapped British aid worker Linda Norgrove in Afghanistan, an investigation has concluded, British Foreign Secretary William Hague said Thursday and The Wall Street Journal reported.

Mr. Hague said that Ms. Norgrove died as a result of fragmentation-grenade shrapnel into her head and chest. The grenade had been thrown at a group of insurgents that had come out of the compound to engage with U.S. troops and who, unbeknown to U.S. forces, had Ms. Norgrove with them.

Ms. Norgrove, 36 years old, was slain on Oct. 8 as American forces stormed the compound where she was being held in the mountains of eastern Afghanistan, and U.K. and U.S. officials initially said her captors had killed her. Soon after, however, American forces said that Ms. Norgrove may have died from a U.S. grenade, and set up a joint U.K. and U.S investigation to look into the matter.

Ms. Norgrove was traveling along a main road in Kunar province when she was abducted in September by a local Taliban faction linked to al Qaeda. She was working with Development Alternatives Inc., one of the largest development contractors working for the U.S. State Department in Afghanistan. Three Afghans abducted with Ms. Norgrove were subsequently released unharmed.

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