PHNOM PENH, Cambodia--The Khmer Rouge’s chief jailer, who admitted overseeing the torture and killings of as many as 16,000 Cambodians while running the regime’s most notorious prison, has died. Kaing Guek Eav, known as Duch, was 77 and had been serving a life prison term for war crimes and crimes against humanity.

He died at a hospital in Cambodia early Wednesday morning, said Neak Pheaktra, a spokesperson for the tribunal in Phnom Penh that handled the trials over the regime’s crimes.

Duch was admitted to Cambodian Soviet Friendship Hospital after developing difficulty breathing Monday at the Kandal provincial prison, said Chat Sineang, chief of the prison where Duch had been transferred from the tribunal’s prison facility in 2013. He added that the body would be examined for a cause of death before being handed to his family.

Duch, whose trial took place in 2009, was the first senior Khmer Rouge figure to face the U.N.-backed tribunal that had been assembled to deliver justice for the regime’s brutal rule in the late 1970s, which is blamed for the deaths of 1.7 million people--a quarter of Cambodia’s population at the time.

The commander of the top-secret Tuol Sleng prison code-named S-21 was one of the few ex-Khmer Rouge who acknowledged even partial responsibility for his actions, and his trial included his own wrenchingly graphic testimony of how people were tortured at the prison.

Men, women and children seen as enemies of the regime or who disobeyed its orders were jailed and tormented there, and only a handful survived.

“Everyone who was arrested and sent to S-21 was presumed dead already,” he testified in April 2009.

Torturers under Duch beat and whipped prisoners and shocked them with electrical devices, Duch admitted to the court, but still he denied accounts from survivors and other trial witnesses that he took part in torture and executions himself. The offspring of detainees were killed to ensure the next generation could not take vengeance. Duch called himself “criminally responsible” for babies’ deaths but blamed his subordinates for battering the young bodies against trees.

He said the prison’s own guards and interrogators were killed for small mistakes and showed rare emotion on the witness stand in June 2009 while speaking of seeing his fellow revolutionaries locked in the cells of his prison. Confessing to betraying his own friends, he said: “That was beyond cowardly.”

When a guilty verdict was finally delivered against him in July 2010, he was sentenced to 35 years, shortened to just 19 due to time served. The judges said they considered the Cold War context of the atrocities and Duch’s cooperation and expressions of remorse, however limited. But outraged survivors feared he could one day walk free. On appeal, the sentence was lengthened in 2012 to life in prison for his “shocking and heinous” crimes against the Cambodian people.

Like many key members of the Khmer Rouge, Duch was an academic before he became a revolutionary. The former math teacher joined Pol Pot’s movement in 1967, three years before the U.S. started carpet-bombing Cambodia to try to wipe out Northern Vietnamese troops and Viet Cong inside the border.

The Khmer Rouge seized power in 1975 and immediately attempted a radical transformation of Cambodia into a peasant society, emptying cities and forcing the population to work on the land. They backed up their rule with ruthless elimination of perceived enemies, and by 1976, Duch was the trusted head of its ultimate killing machine, S-21.

Tribunal judges said he signed off on all executions there and was often present when interrogators used torture to extract confessions, including pulling out prisoners’ toenails, administering electric shocks, and waterboarding. Despite his denials, the judges said he had at times taken part in the torture and executions himself.

The torture and executions that took place at Tuol Sleng were routinely recorded and photographed, and when the Khmer Rouge were forced from power in 1979, the thousands of documents and film negatives left at the prison became proof of the regime’s atrocities.

Duch fled, disappearing for almost two decades in northwestern Cambodia and converting to Christianity until a chance discovery by a British journalist in 1999 led to his arrest.

Duch has several times asked for forgiveness, even offering at one point to face a public stoning. But his surprise request on the final day of the trial to be acquitted and freed left many wondering if his contrition was sincere.