Cambodia on Tuesday marked 50 years since the country’s communist Khmer Rouge launched a four-year reign of terror that killed an estimated 1.7 million people.

On Cambodia’s annual Day of Remembrance, about 2,000 people attended a ceremony honouring the victims of the Khmer Rouge genocide at Choeung Ek, a site of one of the most notorious “killing fields” — about 15 kilometres south of the capital Phnom Penh.

Dozens of student actors from a Phnom Penh art school re-enacted brutalities under the Khmer Rouge, which held power from 1975 to 1979, when an estimated one quarter of Cambodia’s population was wiped out due to torture, starvation, executions, and misrule.

The re-enactment was held near a memorial displaying victims’ skulls and mass graves where thousands of others were buried.

“When I come here, it reminds me, and I will never forget this Khmer Rouge regime because it was extremely cruel and barbaric,” said 71-year-old Nhem Sovann, a Phnom Penh resident who lost six family members — her parents-in-law and four siblings — to the regime. She was put to work farming a rice field in the western province of Pursat.

“I saw with my own eyes that even children were taken and had their heads smashed against the trunk of a coconut tree,” she said.

The Day of Remembrance is a learning experience for younger Cambodians, such as 23-year-old art student Pen Kunthea.

She said she first learned about the regime from her parents and from her studies, and that one of her uncles died from illness during the Khmer Rouge’s time in power.

“I feel scared, and I think the Khmer Rouge made our people afraid and I don’t want that regime to happen again,” she said.

The Khmer Rouge captured Phnom Penh on 17 April 1975 and immediately herded almost all the city’s residents into the countryside, where they were made to toil in harsh conditions.

The regime ruled until 1979, when it was driven from power by an invasion from neighbouring Vietnam, which forced its ruler Pol Pot into hiding.

In 1984, a new Cambodian government installed by the Vietnamese declared 20 May — the day the genocide was said to begin — as a “National Day of Hatred” for people to express their anger against the Khmer Rouge and its backers.

At the time, the Khmer Rouge were still trying to regain power by fighting a guerrilla war from the countryside. They were finally subdued in 1997.

In 2018, the day was officially redesignated the National Day of Remembrance, with an emphasis on honouring the victims.

On Tuesday, Prime Minister Hun Manet urged all Cambodians join in preserving and protecting peace.

“Even though these tragic events have passed, and the Cambodian people have been living in peace, political stability, and full of development in all fields, we must not let go or forget this bitter past,” he said in a statement.

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 Neth 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 UN-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.

As commander of the top-secret Tuol Sleng prison code-named S-21, Duch 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.

The tribunal since Duch’s trial has convicted two top echelon Khmer Rouge leaders, while two other defendants died before their trials could be completed. The regime’s No. 2 leader Nuon Chea died during his appeals process. The tribunal, established in 2004 by an agreement between the U.N. and the Cambodian government, has cost more than $360 million.

The other whose appeal is under consideration, former head of state Khieu Samphan, almost certainly will be the last one to face trial, due to the Cambodian government’s opposition to any more prosecutions.

The communist regime’s top leader, Pol Pot, died in 1998 as a prisoner of his comrades in what had shrunk to a spent force of jungle-based guerrillas.

Youk Chhang, head of the Documentation Centre of Cambodia, which has collected voluminous archives about the country’s tragedy, said Duch’s death “is a reminder to us all to remember the victims of the Khmer Rouge. And that justice remains a difficult road for Cambodia.”

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 in the country they renamed Democratic Kampuchea. 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.