Mr. Colburn, right, and Hugh Thompson meet with schoolchildren during a visit to My Lai in 1998. (Claro Cortes IV/Reuters)

Lawrence Colburn, an Army helicopter gunner who along with two comrades intervened in the U.S. slaughter of unarmed villagers in My Lai during the Vietnam War, an act of heroism for which he received the Soldier’s Medal three decades after the fact, died Dec. 13 at his home in Canton, Ga. He was 67.

The cause was liver cancer, said his wife, Lisa Colburn.

Mr. Colburn was an 18-year-old Army specialist when he witnessed the massacre of hundreds of civilians in the hamlet of My Lai, in the Quang Ngai Province of South Vietnam, as it was underway on March 16, 1968. Nearly a half-century later, the killings loom in memory as one of the most shameful episodes in U.S. military history, as well as a tragic demonstration of the searing psychological effects of war on those called to fight.

“The massacre was horrific, and the cover-up was horrific, and this kid did something special,” said journalist Seymour M. Hersh, who won a Pulitzer Prize for exposing the massacre. “He’s as much of a hero as anybody I can think of.”

Along with his pilot, Hugh Thompson Jr., and their crew chief, Glenn Andreotta, Mr. Colburn set out that morning on a routine aerial reconnaissance mission. When the men spotted wounded civilians — casualties in U.S. ground efforts to root out the enemy Viet Cong — they dropped colored smoke to mark the victims’ location for the U.S. medical units that they assumed were on the way.

My Lai massacre survivor Do Ba places incense at his family's grave as Mr. Colburn looks on during a commemoration of the 40th anniversary of the killings. (Chitose Suzuki/Associated Press)

As the men continued their surveillance, they observed that the injured civilians were not being aided, and were instead being killed.

“It became obvious to us what was happening when we lingered by one of the bodies that we’d marked,” Mr. Colburn said in an interview on the PBS program “American Experience.” “It was a young female with a chest wound, but she was still alive. . . . We saw a captain approach the woman, look down at her, kick her with his foot, step back, and [he] just blew her away right in front of us.”

“She was no threat,” he said. “There was no reason to do that.”

Mr. Colburn credited Thompson with taking the initiative to step in. “We were ready to face the consequences,” he said. “It was so obviously wrong. When you see babies machine-gunned, you must intervene.”

Thompson first landed the helicopter near an irrigation ditch where women, children and the elderly were sheltering. Thompson approached a soldier standing over the group and ordered him to help the civilians out of the ditch. The soldier agreed but began executing the group after the helicopter took off, Mr. Colburn said.

Later, from their air, Thompson and his crew identified a group of Vietnamese hiding in a bunker and a unit of U.S. soldiers advancing on them. Thompson again landed the helicopter and confronted the lieutenant, then called on Mr. Colburn and Andreotta for help. Thompson said that he would personally remove the Vietnamese from the bunker to safety, and that if the Americans fired on them, Mr. Colburn and Andreotta should shoot them.

“The first thing I thought of was my mother,” Mr. Colburn told PBS. “ ‘Oh my God, Mom, get me out of here.’ But, trying to think what she would want me to do, and she would want me to do exactly what Mr. Thompson was doing.”

The Soldier’s Medal, bestowed on Mr. Colburn in 1998, credited him with providing cover as Thompson evacuated the civilians. Mr. Colburn told PBS that he did not train his gun on his fellow Americans, and that he did not think he could have fired on them unless they shot at him first. The soldiers did not resist, and at least 10 Vietnamese were safely evacuated.

Later, Mr. Colburn returned with Thompson and Andreotta to the irrigation ditch, where they rescued an injured boy from under a mound of corpses. Mr. Colburn’s efforts on the ground with Thompson and Andreotta and their radio reports of the killings were credited with bringing about a cease-fire.

Mr. Colburn testified at courts-martial and other investigations into the massacre. Only one person, Lt. William L. Calley Jr., who contended that he was following orders to take out the enemy, was convicted in the case. The Atlanta Journal-Constitution reported that Mr. Colburn was denigrated as a hippie for his testimony, particularly after his own court-martial later that year for possession of marijuana.

“We’re not popular,” he told the Baltimore Sun years later. “The Army doesn’t admit mistakes.”

Mr. Colburn initially received the Bronze Star Medal, a decoration recognizing valor in a combat zone. He regarded the award as indicative of the military’s denial about the My Lai episode, which did not involve enemy combatants at all.

Shortly after receiving the Soldier’s Medal — honoring valor in action not involving hostile forces — Mr. Colburn and Thompson returned to My Lai to mark the 30th anniversary of the massacre. Andreotta had been killed in action 23 days after events at My Lai.

“Why they fired on those people,” Mr. Colburn said during his return visit, “I still don’t know.”

Lawrence Manley Colburn was born in Coulee Dam, Wash., on July 6, 1949. His father was an engineer who worked on dams, and his mother was a homemaker.

Mr. Colburn joined the Army at 17. After his time in Vietnam, he later worked in commercial fishing in Alaska, managed a ski resort shop and operated a medical supplies business. He said that for decades after the war, he dreamed about what he had seen at My Lai.

Survivors include his wife of 31 years, the former Lisa Cale, and a son, Connor Colburn, both of Canton; and three sisters. Thompson died in 2006.

Mr. Colburn saw parallels between the My Lai massacre and more recent events including the 2005 incident in Haditha, Iraq, where U.S. Marines who had lost a comrade to a roadside bomb were accused of killing two dozen Iraqi civilians.

“I can also understand the guys in uniform,” he told a military publication in 2006. “I’ve gone out seeking revenge myself. When you see a buddy taken down, something primal inside you clicks, and you go out and take revenge. And it’s a vicious cycle.”

These atrocities are “stark reminders to a weary public of what war does to people — both the victims and the perpetrators,” he wrote in an online commentary in 2012. “War destroys people.”