At first Thompson and his crew, Lawrence Colburn and Glen Andreotta thought the wounded Vietnamese they saw below them were the result of artillery fire.

They dropped a green flare near a wounded civilian, expecting the infantry to help. Instead, Captain Medina of Charlie Company walked over and shot her in the head. /2

Horrified, Thompson spotted similar scenes unfolding at an irrigation ditch nearby. He immediately landed his helicopter, attempting to stop the murder.

“These are human beings!" He yelled at the Lieutenant there, leading the killing. "Unarmed civilians sir!”

The Lieutenant told Thompson he was stepping outside his authority and ordered him back into his helicopter. /4

From this point on, it was clear to Thompson and his crew that nobody was going to restrain the ground forces.

Thompson had no direct radio to those on the ground, nor to their commanders back at base, so began demanding the other aerial forces present - who did have radio communications with them - intervene.

They stayed silent. /5

While Thompson continued to demand intervention, his crew spotted 2nd Platoon Charlie Company, under Lieutenant Brooks closing on a group of fleeing women, children and old men to the north.

Thompson immediately threw his scout helicopter round and towards them. He grounded it between the advancing soldiers and the terrified civilians. /6

Jumping out of the helicopter with only his side arm, he turned to Colburn, who was on the helicopter's fixed machine gun, and issued him an order Colburn later said he would never forget.

Thompson told him that if Charlie Company fired on Thompson OR the civilians...

“Open up on ‘em. Blow ‘em away.”

With Thompson and the helicopter now blocking Brooks' path to the civilians, Brooks demanded Thompson and his crew move.

He refused. /7

For the next 20 minutes, an angry standoff took place on the ground. The helicopter crew starred down 2nd Platoon, while Thompson continued to beg and swear over the radio at the other units in the air, demanding help.

Eventually, two helicopters broke away and landed behind them, ushering the civilians onboard to safety.

Thompson recognised them as friends of the crew. /8

Low on fuel, Thompson' realised the only chance of stopping the massacre was direct intervention from Army HQ at LZ Dottie. They took to the air and aimed the helicopter towards it, hoping to get word out and the killing stopped.

Passing over the irrigation ditch again, Andreotta saw movement below among the dead.

They landed. Thompson covered his crew with the machine gun as they pulled a 3yr old boy out alive. /9

At LZ Dottie, Thompson stormed into HQ loudly announcing to all present what was happening at My Lai.

"It's mass murder out there. They're rounding them up and herding them in ditches and then just shooting them."

Other officers told him to calm down and not get involved. He would not be quieted. Despite efforts to stop him, he forced his way into operational command. /10

There, Thompson raged loudly that the American soldiers on the ground were acting no better than Nazis. It caused enough of storm that, spooked, the Task Force’s commander radioed through an order that all operations at My Lai should cease immediately. Any killing should stop.

Now fully refuelled, Thompson and his crew returned to make sure it did. /11

Whereas others tried to whitewash what had happened, Thompson’s post-operation report into the incident held back nothing.

My Lai was now becoming uncomfortable for the US forces. Rumours were circulating. Thompson's account was problematic.

They quietly issued him a Distinguished Flying Cross, awarded for "bravery under crossfire." in hope it would quieten him.

He threw it away. That fire was from his own countrymen killing civilians. /12

Eventually, Thompson was seriously wounded in action and sent back to the US where he continued to serve, but lost contact with his former crewmates Colburn and Andreotta.

Still an officer, under continuing pressure to stay silent and unwilling to break his oath of loyalty to the Army, he remained vocal internally but felt unable to to go public with what he had seen. /13

This changed the moment the first public account of the massacre appeared. Thompson broke cover.

He refused to keep quiet or minimise the crimes committed, despite pressure from press, the military and a hostile Congress determined to cast him as a traitor.

He stood up. He testified at the public enquiry. He was vilified by many. /14

Thompson was forced to watch as those involved in the massacre, including Medina, were publicly cleared and presented as heroes in difficult circumstances.

Meanwhile, Thompson and his family received death threats. Dead animals were left on his porch. He was threatened with court-martial for ordering his crew to draw weapons on Charlie Company.

All this, he was told privately, would go away if he changed his account.

Thompson refused, but was forced to retreat from public life for safety /15

Twenty years later in 1988 Michael Bilton, a British filmmaker, was trying to make a documentary about My Lai. He kept coming across Thompson's name, and his account and eventually managed to track him down.

Bilton asked Thompson if he would go on camera and tell people what happened. Would he risk going through all that again, to tell the truth?

Thompson agreed, without a moment of hesitation. /16