In the course of three hours more than 500 Vietnamese civilians were killed in cold blood at the hands of US troops. The soldiers had been on a "search and destroy" mission to root out communist fighters in what was fertile Viet Cong territory.
Yet there had been no firefight with the enemy - not a single shot was fired at the soldiers of Charlie Company, a unit of the Americal Division's 11th Infantry Brigade.
The 48th Viet Cong Battalion - the intended target of the mission - was nowhere to be seen.
When the story of My Lai was exposed, more than a year later, it tarnished the name of the US army. Most Americans did not want to believe that their revered GI Joe could be a wanton murderer.
My Lai was the sort of atrocity American patriots preferred to associate with the Nazis.
Charlie Company had arrived in Vietnam three months before the My Lai massacre.
By then the US - fighting alongside the South Vietnamese army - was deeply entrenched in war against North Vietnam's communist forces. The United States's had deployed nearly 500,000 soldiers in Vietnam, a commitment which cost it $2 bn every month.
In January 1968 the Viet Cong guerrillas and the regular North Vietnamese Army launched a joint attack on US positions, known as the Tet Offensive. Washington maintained it could win the war, but on the ground morale among its troops was low.
Charlie Company was down to 105 men by mid-March of that year. It had suffered 28 casualties, including five dead. Some of its soldiers had already begun to drift towards brutal tactics for which they appeared to enjoy impunity.
The brief for its March 16 mission was to prise out the Viet Cong, whose elusive troops were thought to be hiding in My Lai - a hamlet of the Son My village.
Two platoons moved in shortly after 8pm in the morning, while a third held back for "mopping up" duties. Both platoons soon splintered and once the shooting started it seemed to spark a chain reaction.
Soldiers went berserk, gunning down unarmed men, women, children and babies. Families which huddled together for safety in huts or bunkers were shown no mercy. Those who emerged with hands held high were murdered.
Some lucky villagers survived the massacre
Some of the 120 or so soldiers opted out of the killing spree, but troop commander Lt William Calley was not one of them. In one incident, Lt Calley ordered two of his men to fire on a group of 60 civilians they had rounded up. When one refused, Calley took over and, standing 10 feet from the crowd, blazed his gun at them.
Elsewhere in the village, other atrocities were in progress. Women were gang raped; Vietnamese who had bowed to greet the Americans were beaten with fists and tortured, clubbed with rifle butts and stabbed with bayonets. Some victims were mutilated with the signature "C Company" carved into the chest.
By late morning word had got back to higher authorities and a cease-fire was ordered. My Lai was in a state of carnage. Bodies were strewn through the village. The death toll totalled 504.
Only one American was injured - a GI who had shot himself in the foot while clearing his pistol.
The Cover Up
It took more than a year for the shocking story of the My Lai massacre to reach the news stands.
Efforts had been made to cover up the atrocity from day one. Charlie Company's Captain Ernest Medina, who was on the ground at My Lai, realised that news of the events could cause trouble.
Despite witnessing at least 100 bodies, when questioned by a superior close to the scene, he maintained that between 20 and 28 civilians had been killed by gunship and artillery fire.
That was also the essence of a report submitted a month later by the commander of the 11th Infantry Brigade, Col Oran K Henderson. 20 civilians had been killed inadvertently, he claimed.
But the rumour mill, that had began turning within days of the My Lai massacre, told a very different story.
Soldier Paul Meadle
One of the soldiers in C company, Paul Meadle, admitted his part in the massacre on television
It took a combination of loose talk and a conscientious GI who harboured ambitions to become a journalist, for the allegations to reach the corridors of power back in Washington.
Ridenhour made it his mission to find out
Ronald Ridenhour, a soldier with the 11th Brigade soldier and also serving in Quang Ngai Province, was sharing a beer with members of Charlie Company when one of them started to boast of their exploits in My Lai.
Ridenhour was revolted but, from then on, made it his mission to substantiate the claims by speaking to other members of the squad.
Back in the US, he set down the allegations in a story which he posted to 30 top names in Washington. General William Westmoreland, who was in overall command of the Vietnamese operation, could not believe his men would engage in mass murder and ordered an immediate inquiry.
Evidence was amassed and the inquiry became a criminal investigation. Lt Calley, commander of the 1st platoon at My Lai, was called back to the US as a potential suspect and in September of 1969 he was charged with 109 murders.
The whitewash was about to begin.
In late 1969 the grisly details of My Lai were unleashed on the public, following a report by the investigative journalist Seymour Hersh.
Around the same time the army commissioned an investigation into the cover-up, which became known as the Peers inquiry.
More than 400 witnesses were questioned and 20,000 pages of testimony taken before the inquiry reported its findings in March 1970. Meanwhile an investigation by the army's Criminal Investigation Division (CID) was also conducted into the crimes committed at My Lai.
The Peers report told a comprehensive story of what had happened on March 16, 1968. The crimes had included murders by individuals and groups, rape, sodomy, maiming and assault of civilians.
The report concluded that both Col Henderson, the brigade commander, and Lt Col Frank Barker, the commanding officer of the task force, had substantial knowledge of the war crime, but did nothing about it.
The Peers inquiry recommended that charges should be brought against 28 officers and two non-commissioned officers involved in the concealment of the massacre.
Charges dismissed and accused found not guilty
But the prospect of prosecutions crumbled . Army lawyers decided only 14 officers should be charged. Only one came to court, and he was acquitted.
A similar pattern emerged in the prosecution of the ground troops who had done the killing at My Lai.
The CID report said there was evidence to charge 30 soldiers with major crimes. 17 had left the army and charges against them were quietly dropped. Elsewhere charges were dismissed or the accused found 'not guilty'.
The judicial process was labelled a whitewash by those who had fought to bring the soldiers of Charlie Company to book.
In the end, Charlie company's commanding officer, Lt Calley, was the only one to be convicted. He was sentenced to life in jail but released three years later after intervention by President Richard Nixon.