By Guy Delauney
BBC News, Phnom Penh
Duch ran a Khmer Rouge prison where almost all inmates were killed
A former Cambodian prison chief accused of presiding over thousands of murders will be the first person to go on trial at country's Khmer Rouge tribunal.
Kaing Guek Eav, better known as Comrade Duch, faces charges of crimes against humanity and war crimes.
He ran Tuol Sleng prison, where detainees were tortured and executed.
As many as two million people are thought to have died during the four years of Khmer Rouge government in the late 1970s.
Two years have passed since work officially started on bringing the former leaders of the Khmer Rouge to justice.
There have been numerous delays and controversies, but the formal indictment of Duch is a sign of the progress the tribunal has made.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Up to two million people thought to have died from starvation, overwork or execution
A spokesman called it "an important moment in the history of the courts".
Putting the former prison chief on trial will give a boost to the credibility of the process.
Duch was in charge of the notorious facility known as S-21 or Tuol Sleng, where about 15,000 prisoners were systematically tortured.
Those who survived the ordeal were sent for execution in the so-called "killing fields".
Officials have indicated that Duch cooperated with the investigating judges and is willing to testify in court.
So the trial should be a chance for survivors of the Khmer Rouge era to hear directly at last from one of the organisation's key figures.
Progress on the judicial side is also a welcome distraction from the tribunal's many problems.
Donors are withholding promised funds because of corruption allegations and local staff have been working without pay.