GAINESVILLE, Fla. (AP) — William L. 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 at a hospice center in Gainesville, Florida, The Washington Post reported Monday, citing his death certificate. The Florida Department of Health in Alachua County didn’t immediately respond to Associated Press requests for confirmation.

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 Vietnam War massacre.

On March 16, 1968, Calley led American soldiers of the Charlie Company on a mission to confront a crack outfit of their 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.

The men were angry: Two days earlier, a booby trap had killed a sergeant, blinded a GI and wounded several others while Charlie Company was on patrol.

Soldiers eventually testified to the U.S. Army investigating commission that the murders began soon after Calley led Charlie Company’s first platoon into My Lai that morning. Some were bayoneted to death. Families were herded into bomb shelters and killed with hand grenades. Other civilians slaughtered in a drainage ditch. Women and girls were gang-raped.

It wasn’t until more than a year later that news of the massacre became public. And while the My Lai massacre was the most notorious massacre in modern U.S. military history, it was not an aberration: Estimates of civilians killed during the U.S. ground war in Vietnam from 1965 to 1973 range from 1 million to 2 million.

The U.S. military’s own records, filed away for three decades, described 300 other cases of what could fairly be described as war crimes. My Lai stood out because of the shocking one-day death toll, stomach-churning photographs and the gruesome details exposed by a high-level U.S. Army inquiry.

Calley was convicted in 1971 for the murders of 22 people during the rampage. He was sentenced to life in prison but served only three days because President Richard Nixon ordered his sentence reduced. He served three years of house arrest.

After his release, Calley stayed in Columbus and settled into a job at a jewelry store owned by his father-in-law before moving to Atlanta, where he avoided publicity and routinely turned down journalists’ requests for interviews.

Calley broke his silence in 2009, at the urging of a friend, when he spoke to the Kiwanis Club in Columbus, Georgia, near Fort Benning, where he had been court-martialed.

“There is not a day that goes by that I do not feel remorse for what happened that day in My Lai,” Calley said, according to an account of the meeting reported by the Columbus Ledger-Enquirer. “I feel remorse for the Vietnamese who were killed, for their families, for the American soldiers involved and their families. I am very sorry.”

He said his mistake was following orders, which had been his defense when he was tried. His superior officer was acquitted.

William George Eckhardt, the chief prosecutor in the My Lai cases, said he was unaware of Calley ever apologizing before that appearance in 2009.

“It’s hard to apologize for murdering so many people,” said Eckhardt. “But at least there’s an acknowledgment of responsibility.”