The Khmer Rouge commander known as ‘Comrade Duch’, Pol Pot’s chief executioner who oversaw the mass murder of at least 14,000 Cambodians at the notorious Tuol Sleng prison, has died. He was 77.

Neth Pheaktra, a spokesman for the Khmer Rouge tribunal that convicted him of war crimes, said: “Duch died this morning at 00:52am, on September 2 at Khmer Soviet friendship hospital. Details of what he died of, I can’t tell.” Duch had been in poor health in recent years.

Duch was a major figure in Pol Pot’s ultra-communist regime which killed an estimated 1.7 million people, a quarter of Cambodia’s population, in the country’s “killing fields’” between 1975 and 1979.

The 77-year-old was in 2010 the first Khmer Rouge commander convicted of crimes against humanity, and was later sentenced to life after a UN-backed tribunal rejected his appeal that he was a junior official following orders.

Duch, whose given name was Kaing Guek Eav, was by the time of his trial a born-again Christian and expressed regret for his crimes. He told a court in 2009: “I am solely and individually responsible for the loss of at least 12,380 lives.”

He ran the notorious Tuol Sleng prison – codenamed “S-21” – in central Phnom Penh, where over four years in the late 1970s, it is reckoned more than 12,000 men, women and children passed through before being murdered by the Khmer Rouge. Most were tortured into confessing crimes they couldn’t possibly have committed before being loaded on to trucks and driven to the notorious killing fields of Choeung Ek, where they were bludgeoned to death with ox-cart axles.

The guards were instructed to “smash to bits” traitors and counter-revolutionaries. For the Khmer Rouge, that could mean anyone from school teachers to children, to pregnant women and “intellectuals” identified as such for wearing glasses.

The prison, a former Phnom Penh high school, still stands as a memorial to the atrocities committed inside.

Duch – a former maths teacher – had an obsessive eye for detail and kept his school-turned-jail meticulously organised. “Nothing in the former schoolhouse took place without Duch’s approval. His control was total,” wrote photographer and author Nic Dunlop, who found Duch in 1999 hiding near the Thai border, two decades after the Khmer Rouge fell.

“Not until you walk through the empty corridors of Tuol Sleng does Stalin’s idiom that one death is a tragedy – a million a statistic, take on a terrifying potency,” Dunlop wrote in his account of Duch and his atrocities, The Lost Executioner.

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. “He was cooperative, he spoke to the court frankly. He 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”.