PHNOM PENH -- The Khmer Rouge commander known as ‘Duch’, Pol Pot’s premier executioner and security chief who oversaw the mass murder of at least 14,000 Cambodians at the notorious Tuol Sleng prison, died on Wednesday. He was 77.

Kaing Guek Eav or ‘Duch’ was the first member of the Khmer Rouge leadership to face trial for his role within a regime blamed for at least 1.7 million deaths in the “killing fields” of Cambodia from 1975 to 1979.

Duch died at 00:52 a.m. (1752 GMT on Tuesday) at the Khmer Soviet Friendship Hospital in Phnom Penh, Khmer Rouge tribunal spokesman Neth Pheaktra said. He gave no details of the cause, but Duch had been ill in recent years.

In 2010, a U.N. tribunal found him guilty of mass murder, torture and crimes against humanity at Tuol Sleng prison, the former Phnom Penh high school which still stands as a memorial to the atrocities committed inside.

He was given a life sentence two years later after his appeal that he was just a junior official following orders was rejected. Duch - by the time of his trial a born-again Christian - expressed regret for his crimes.

Under Duch’s leadership, detainees at Tuol Sleng prison, codenamed “S-21”, were ordered to suppress cries of agony as Khmer Rouge guards, many of whom were teenagers, sought to extract confessions for non-existent crimes through torture.

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.

Beneath Tuol Sleng’s chaotic facade, Duch - himself 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.

At S-21, new prisoners had their mugshots taken. Hundreds are now on display within its crumbling walls.

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.

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”.