MY LAI, Vietnam — The shudder of artillery fire woke the boy at 5:30 a.m. Three American soldiers appeared at his family’s home a couple of hours later and forced the mother and five children into their bomb shelter, a structure almost every rural home had during the Vietnam War, to keep residents safe.

One soldier set fire to the family’s thatched house while the others tossed grenades into the shelter. Protected under the torn bodies of his mother and his four siblings, 10-year-old Pham Thanh Cong was the only survivor.

It was March 16, 1968, 50 years ago. The American soldiers of Charlie Company, sent on what they were told was a mission to confront a crack outfit of their Vietcong enemies, met no resistance, but over three to four hours killed 504 unarmed civilians, mostly women, children and elderly men, in My Lai and a neighboring community. Vietnamese refer to the greater village where the killings occurred as Son My.

“We started hearing the screaming and moaning from our neighbors, which were followed by gunfire and grenade explosions, then the screaming and moaning stopped, and my mother knew that the American soldiers had killed people,” Cong said. “I was covered with the flesh and hair of my mother and sisters and brother.”

Knocked unconscious with injuries to his head and wounds on his torso from grenade fragments, Cong was saved that afternoon when his father came to retrieve the bodies.

The My Lai massacre was the most notorious episode in modern U.S. military history, but not an aberration in America’s war in Vietnam.

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

An official policy of free-fire zones – from which civilians were supposed to leave upon being warned – and an unofficial code of “kill anything that moves” meant Vietnamese were constantly at risk. Estimates of civilians killed during the U.S. ground war in Vietnam from 1965 to 1973 are generally 1 million to 2 million.

The average U.S. soldier could not be sure who the enemy was, rarely encountering one directly. They were targeted by land mines, booby traps, snipers. Quang Ngai province, where My Lai is located, was a hive of communist military activity.

Two days before the massacre, a booby trap killed a sergeant, blinded a GI and wounded several others on a Charlie Company patrol.

Soldiers later testified to the Army investigating commission that the bloodletting began quickly when Lt. William L. Calley Jr. led Charlie Company’s first platoon into My Lai that morning. One elderly man was bayoneted to death; another man was thrown alive into a well and killed with a hand grenade. Women and children were herded into a drainage ditch and slaughtered. Women and girls were gang-raped.

“They went in with blood in their eyes and shot everything that moved,” recalled Hugh Thompson Jr., an army helicopter pilot who flew support for the mission in My Lai and – along with his two-man flight crew – are the only servicemen to have actively intervened to try to stop the killing. They evacuated a handful of Vietnamese civilians, and Thompson also was one of several soldiers who became whistleblowers and eventually brought the outrage to public attention.

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.

Calley, the only person convicted in the massacre, has avoided speaking about it with apparently just one exception. In 2009, 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. “I feel remorse for the Vietnamese who were killed, for their families, for the American soldiers involved and their families. I am very sorry.”