Comrade Duch, who ran the most notorious prison in Cambodia and was convicted of crimes against humanity, has died.
He was serving a life term after being sentenced by a UN-backed court.
Kaing Guek Eav, better known as Comrade Duch, ran the notorious Tuol Sleng prison, in which thousands were tortured and murdered in the 1970s.
As many as two million people are believed to have been killed under the Khmer Rouge, a Maoist regime that controlled Cambodia until Vietnam intervened.
Duch died on Wednesday in hospital, aged 77, a spokesman in the capital Phnom Penh said, without giving further details.
While chief at Tuol Sleng, also known as S-21 prison, Duch kept an enormous archive of photos and documents that helped prosecutors trace the final months of thousands of inmates’ lives.
It is thought that at least 15,000 men, women and children deemed enemies of the regime passed through the gates of the prison.
Most of them were tortured, forced to confess to fictitious crimes against the Khmer Rouge and then put to death at the so-called “killing fields” just outside the capital.
Fewer than a dozen prisoners survived Tuol Sleng.
“Nothing in the prison took place without Duch’s approval. His control was total,” wrote photojournalist Nic Dunlop, who found him in 1999 hiding near the Thai border.
In Cambodia’s capital, Phnom Penh, people were subdued about the war criminal’s death.
One resident told Reuters, “He deserved to serve more prison terms. But now he has died, I can forgive him, and his case is finished.”
Norng Chan Phal, one of the few people to have survived S-21, was a boy when he and his parents were sent to Duch’s prison and interrogated on suspicion of having links to the Khmer Rouge’s mortal enemy, Vietnam.
His parents were tortured and killed but Chan Phal survived to give testimony at Duch’s trial in 2010.
“Duch apologised to all S-21 victims and asked them to open their hearts. He apologised to me too,” Chan Phal told Reuters.
“He apologised. But justice is not complete.”