Like all crimes, it was all supposed to be conducted in secret.
In September 2008, as Sri Lankan government forces pushed the fighters of the Tamil Tigers further and further back into the Tamil homelands of the north, the government ordered the UN to evacuate their last few international workers from Kilinochchi, the Tigers' de facto capital.
The reason, they said, was they could no longer guarantee their safety.
The real reason was far less honorable: They did not want any witnesses to what was coming.
One of the UN staff, communications Officer Benjamin Dix, recalls how distressed and angry they felt. A mood which was not improved by the celebratory party the UN threw for them when they escaped the war zone.
"I remember feeling pretty disgusted by that party. I didn't see that there was anything there to celebrate. What we had actually done was complete abandonment of our duty of protection of civilians in a conflict situation," he said.
The next day Dix resigned from his post. But even he had no idea just how catastrophic that abandonment was, how awful was the disaster that was about to befall the people left behind.
With the UN out of the region, with international media excluded and local journalists and critics silenced, exiled, disappeared or in fear of their life, the government felt ready to launch the final offensive.
On January 2, 2009, Kilinochchi fell. Between 300,000 and 400,000 civilians were on the run, fleeing further into the Tiger-held territory. But they were fleeing into a terrible trap – a trap which would see tens of thousands of them die, mostly (as a UN panel of experts later concluded) as a result of targeted government shelling.
Back in the Sri Lankan capital of Colombo, the increasingly autocratic regime of President Mahinda Rajapaksa and his brother, the Defense Secretary Gotabaya Rajapaksa, was determined to finish the Tigers off. As the then UN Under-Secretary General for Humanitarian Affairs, Sir John Homes told me: "They were not going to let anybody stop them do that. Either the international community, the media or the fear of humanitarian issues of civilian casualties. And that's the way it worked out."
And if any local journalists were thinking of challenging that plan, they were about to receive a painful reminder of what the consequences might be.
Soon after the fall of Kilinochchi, the founding editor of the Colombo Sunday Leader, the Sinhalese writer, Lasantha Wickrementunge, wrote an article attacking the government's military triumphalism and commitment to a military solution to the Tigers 26-year insurgency. It was not an easy article to write; he had once been a personal friend and admirer of the president.
A few days later, as Wickrementunge was driving to work, he was ambushed and executed by four unknown assailants on motorbikes.
After his death his newspaper published a front page editorial he had written in anticipation of his own murder. It was addressed to his former friend, the president. "For all the dreams you had for our country in your younger days . . . you have trampled on human rights, nurtured unbridled corruption and squandered public money like no other president before you."
And he concluded:
"When finally I am killed, it will be the government that kills me."
But pleas concerning this dead journalist had no effect.
To the regime in Colombo, it must have seemed like all the elements were in place: There was no one left to witness what was about to happen.
Over the next few months those 300,000 to 400,000 civilians were herded into a smaller area of land, trapped with the increasingly desperate forces of the Tamil Tigers who were themselves committing war crimes, even shooting at any civilians who tried to escape.
But it was the government which killed most. They declared what they called "No Fire Zones," and encouraged the civilians to gather there before shelling them in sustained barrages that were to last for months.
But the problem for the government – the problem which, if justice is to be served, will come back to haunt them – is that there were witnesses.
Those witnesses were the victims and the perpetrators themselves. Civilians, Tigers, even government soldiers, who would record everything that happened on small cameras and mobile phones.
For the last three years – with the help of Channel 4 News in the UK, which first broke the story of some of the most brutal executions – we've been compiling that footage, analyzing and authenticating it. It is powerful evidence of terrible war crimes and crimes against humanity – evidence of summary execution, of torture, of sexual violence as a weapon of war, and evidence of the systematic targeting of innocent civilians.
That work has resulted in two television documentaries already and is culminating in the full length feature documentary which we intend to release at the United Nations Human Rights Council in March 2013.
The anguished voices of the dead, the dying and those left behind might yet be heard – and justice may yet catch up with Sri Lanka's war criminals.