Duch was a senior figure in Pol Pot's tyrannical communist regime and ran the notorious Tuol Sleng S-21 torture prison in Phnom Penh, where at least 14,000 people died.

At least 1.7 million people -- nearly a quarter of Cambodia's population -- died from execution, disease, starvation and forced labor under the Khmer Rouge regime that ruled the country between 1975-1979.

Many of those killed were intellectuals or trained professionals -- people considered counter-revolutionaries by the Khmer Rouge leadership bent on turning Cambodia into a purely agrarian society through ruthless social engineering policies.

Duch was the first Khmer Rouge commander to be convicted of war crimes, crimes against humanity and grave breaches of the 1949 Geneva Convention by a United Nations-backed tribunal in 2010. He was handed a life sentence in 2012 after losing an appeal in which he argued that he was just following orders of senior leaders of the Khmer Rouge regime.

At his trial, Duch -- then a born-again Christian -- pleaded guilty to his crimes and apologized to the victims' families, asking for their forgiveness.

His charges were heard at the Extraordinary Chambers in the Courts of Cambodia (ECCC) -- a special UN-backed tribunal that was formed in 2006 to prosecute senior Khmer Rouge leaders and other regime figures responsible for especially heinous acts.

The tribunal began its work in 2007 after a decade of on-and-off negotiations between the United Nations and Cambodia over the structure and functioning of the court.

In 2018, almost four decades after the collapse of Pol Pot's brutal regime, the tribunal ruled that the Khmer Rouge committed genocide.

A former school teacher, Duch became head of the Santebal, which was in charge of internal security and operating prison camps under the Khmer Rouge, according to the Cambodia Tribunal Monitor, a group of academic and nonprofit organizations.

Known by the code name S-21, the former high school of Tuol Sleng became the Khmer Rouge's secret prison and the most potent symbol of its brutality.

At the prison, men, women and children were shackled to iron beds and tortured before they were beaten to death, prosecutors said. Few people taken there made it out alive. Many inmates ended their days along with tens of thousands others in the Killing Fields of Choeung Ek.

"I thought that was the end of my life," one survivor told CNN in 2008. "In my room people kept dying, one or two every day."

The prison was turned into a memorial site documenting the horrors and atrocities of the regime. One building has been preserved exactly as the Vietnamese invaders found it in early 1979, down to the bloodstains on the floor and the implements of torture left on the bed frames. In another hang black and white portraits of the prisoners -- photos taken by the regime -- who died there.