An ex-Khmer Rouge prison chief has been charged with crimes against humanity by a UN-backed tribunal in Cambodia.
Kang Kek Ieu, also known as Duch, was in charge of the notorious S21 jail in the country's capital, Phnom Penh.
Duch is the first of five suspects whom prosecutors have asked the tribunal to investigate over their role in the brutal Khmer Rouge regime.
More than a million people are thought to have died during the four years of Khmer Rouge rule between 1975-79.
Judges spent several hours interviewing Duch on Tuesday before formally filing charges against him.
"The co-investigating judges of the Extraordinary Chambers in the Courts of Cambodia have charged Kang Kek Ieu, alias Duch, for crimes against humanity and have placed him in provisional detention," tribunal judges said in a statement.
Duch was not among the top level of Khmer Rouge leaders but he has become one of its most notorious members, according to the BBC's Guy De Launey in Phnom Penh.
He ran S21, a notorious jail where about 1,400 men, women and children were kept, and many of them brutally tortured.
A museum at the site illustrates in graphic detail what happened to the inmates, many of whom were executed at the so-called Killing Fields outside Phnom Penh.
Duch was also the Khmer Rouge cadre who took charge of French anthropologist Francois Bizot, who recorded his 90 days captivity in a best-selling book, The Gate.
The UN-backed tribunal has taken years to get off the ground.
But by charging Duch, the judges are sending out a clear message that the special courts are now operational and moving more quickly than many people expected, our correspondent says.
Survivors have welcomed the charges, but they have also expressed doubts about whether other, more senior Khmer Rouge leaders will ever be brought to justice.
Duch had been in military custody for eight years and it was a simple matter to transfer him to the special courts. But other Khmer Rouge figures have been living freely in Cambodia - and may prove more difficult to track down.
Among those thought to be on the tribunal's list of suspects are "Brother Number Two" Nuon Chea, former president Khieu Samphan and Khmer Rouge foreign minister Ieng Sary.
But the two most senior Khmer Rouge leaders will never be brought to trial.
"Brother Number One" Pol Pot, the founder and leader of the regime, died in a camp along the border with Thailand in 1998, and Ta Mok, the regime's military commander and one of Pol Pot's most ruthless henchmen, died last year.
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