William L. Calley Jr., who as a young Army lieutenant during the Vietnam War was the only American convicted in the murder of hundreds of unarmed, unresisting Vietnamese civilians in the atrocity known as the My Lai Massacre, died on April 28 in hospice in Gainesville, Fla, according to Social Security Administration records. He was 80.

The cause of his death is not publicly available. Family members of Lieutenant Calley did not immediately respond to requests for additional information. His death was first reported by The Washington Post.

His death record was found through data shared from the Social Security Death Master File to Lexis Nexis.

Nearly 56 years after the killings of as many as 500 women, children and older men by Americans who attacked with automatic weapons, grenades and bayonets; raped girls and women; mutilated bodies; killed livestock, and burned the village, My Lai (pronounced Mee Lye) still reverberates as one of the worst outrages of a brutal and divisive war.

On the morning of March 16, 1968, Second Lieutenant Calley, a 24-year-old platoon leader who had been in Vietnam just three months, led about 100 men of Charlie Company into My Lai 4, an inland hamlet about halfway up the east coast of South Vietnam. The Americans moved in under ambiguous orders, suggesting to some that anyone found in the hamlet, even women and children, might be Vietcong enemies.

While they met no resistance, the Americans swept in shooting. Over the next few hours, horrors unfolded. Witnesses said victims were rousted from huts, herded into an irrigation ditch or the village center and shot.