The Khmer Rouge commander known as “Comrade 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 Comrade 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 00.52am (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 UN 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 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.