William Laws Calley Jr., who as an Army lieutenant led the U.S. soldiers who killed hundreds of Vietnamese civilians in the My Lai massacre, the most notorious war crime in modern American military history, has died. He was 80.

Calley died on April 28, according to his Florida death record, which said he had been living in an apartment in Gainesville. His death was first reported by The Washington Post on Monday, citing his death certificate.

Calley had lived in obscurity in the decades since he was court-martialed and convicted in 1971, the only one of 25 men originally charged to be found guilty in the massacre that helped turn American opinion against the war in Vietnam.

On March 16, 1968, Calley led American soldiers of the Charlie Company on a mission to confront a crack outfit of Vietcong enemies. Instead, over several hours, the soldiers killed 504 unresisting civilians, mostly women, children and elderly men, in My Lai and a neighboring community.