JUDY WOODRUFF: Next tonight, the deadly and gruesome end to a civil war in South Asia.
Ray Suarez begins with some background.
And a warning: Some of the documentary material includes disturbing images.
RAY SUAREZ: It's been just two years since some semblance of peace returned to Sri Lanka, once famed for its tea and beaches. For more than 25 years, beginning in 1983, a civil war raged in the north and east of the tropical island nation off southern India. Militant members of the Tamil minority, the so-called Tamil Tigers, fought a guerrilla war for independence against a government run by the Sinhalese majority.
The Tigers were accused of dozens of suicide bombings in the capital, Colombo, during the 1990s, and human rights groups reported atrocities by both sides during the conflict. The U.N. has reported at least 40,000 civilians, more than half the toll for the entire war, died in the final months of the fighting, in late 2008 and 2009.
Now Britain's Independent Television News has charged that Sri Lankan government forces herded thousands of Tamil civilians into a no-fire zone and then opened fire on them.
Jon Snow narrated the ITN documentary "Sri Lanka's Killing Fields."
JON SNOW: There were between 300,000 and 400,000 people living in the no-fire zone. Satellite and drone footage meant the U.N. and other powers knew a great deal about what went on throughout the war in the no-fire zones. In their Colombo H.Q., the U.N. monitored what happened.
Was it safe in the no-fire zones?
GORDON WEISS, former United Nations spokesman in Sri Lanka: No, it wasn't. The no-fire zone was taking significant amounts of shelling from the government of Sri Lanka. And it was killing civilians.
JON SNOW: The U.N. special panel report published this April found that government heavy artillery was constantly retargeted in the so-called no-fire zones.
Yet, the Sri Lankan government continued to insist it was engaged in a humanitarian rescue operation, with a policy of zero civilian casualties. In April, the government adopted a new approach. They focused a lethal barrage of heavy shelling along a line stretching back from a temporary hospital in Putumattalan, splitting off a section of the zone.
GORDON WEISS: The government cut the sand spit in half, so that 80,000 to 100,000 civilians flooded out and were shipped off to internment camps inside government-held territory.
JON SNOW: This is government footage of some of those fleeing civilians before they were taken into custody by the army. But no one knows how many had died in the assault.
RAY SUAREZ: The other serious charges against the government in the program come from footage too graphic for us to show. The video was apparently shot by government soldiers themselves, and includes extensive evidence of rape, torture and extrajudicial killings of captive Tamil soldiers.
JON SNOW: As thousands of terrified survivors struggled to escape the slaughter at the end of Sri Lanka's civil war in May 2009, triumphant government forces made sure no international observers were allowed anywhere near.
The reason is now clear. Whilst the world was shown these official images of government forces meeting triumphantly on the beach, other members of those same forces were committing a series of horrific war crimes.
The reason we know about these crimes is that some of them were recorded on mobile phones as grotesque war trophies.
RAY SUAREZ: British Prime Minister David Cameron called today for an investigation into the allegations. His government said the documentary shows convincing evidence of violations of human rights in Sri Lanka.