I visited the Killing Fields in Cambodia. They have haunted me ever since.

From 1975 to 1979, estimates of up to 3 million people were killed by the Khmer Rouge Communist Party. Cambodians were tortured, executed or died of disease, overwork and starvation. Almost a quarter of the country’s population perished. The mass killings were part of a state-sponsored Cambodian genocide. Dith Pran, a Cambodian journalist, coined the term “the killing fields” to describe the piles of bodies he passed during his escape.

The Khmer Rouge regime came to power promising peace after years of civil war. They arrested and executed those suspected of connections with the former or foreign governments, as well as professionals and intellectuals. Anyone deemed an enemy of the state was executed.

The Khmer Rouge emptied cities, confiscated property, abolished money and religion, and brutal laws sent the population to work the land under slave-like conditions. Susan Cook, director of the Cambodian Genocide Program at Yale University, said that the driving motivation for the Khmer Rouge was that they “thought they could reorganize a society to bring it backwards in time to a state of agrarian purity.” The Khmer Rouge tried to take Cambodia back to the Middle Ages.

I visited Tuol Sleng, otherwise known as S-21, a prison and torture site housed in a former high school in Cambodia’s capital. One of almost 200 such prisons, Tuol Sleng is now a chilling museum, with hundreds of stark photographs of individuals murdered and weapons used to torture them. Its archives have been registered in UNESCO’s Memory of the World program. Some survivors had tables set up on the grounds selling their memoirs about their time in the prison. Their grim faces spoke volumes about their suffering and trauma.

Those executed were dumped into mass graves. There are more than 20,000 of these graves in Cambodia. Many mass graves are visible above ground, and numerous grave sites have not been excavated. Bones and clothing surface after heavy rainfalls due to the large number of bodies still buried in shallow mass graves.

Some skulls were collected and displayed in stacks at Choeung Ek Genocidal Center, on the outskirts of Phnom Penh, in a memorial serving as a reminder of the dark chapter in the country’s history. On the memorial is a sign that reads, “Would you please kindly show your respect to the many million people who were killed under the genocidal Pol Pot regime.” Bone fragments and bloodied clothes sit in containers around Choeung Ek’s grounds for visitors to observe. Some appeared to be freshly excavated arm and leg bones. Signs around the site read, “Mass grave of 450 victims,” “Mass grave of 166 victims without heads,” “Piece of bones remaining after excavation in 1980” and “Mass grave of more than 100 victims, children and women whose majority were naked.”

A 1984 biographical film about the Khmer Rouge regime, The Killing Fields, based on the experiences of two journalists, Cambodian Dith Pran and American Sydney Schanberg, brought more of the world’s attention to the Khmer Rouge’s atrocities. The film received seven Oscar nominations and won three, including Best Supporting Actor for Haing S. Ngor, a physician who had no previous acting experience and who, himself, survived three terms in concentration camps. He used his medical knowledge to help him survive.

An invasion by neighboring Vietnam in 1979 toppled the regime after a series of violent battles on the countries’ border. Just as he was to face the possibility of trial on the world’s stage, Pol Pot, the Khmer Rouge dictator, died of heart failure in 1998 while imprisoned. Cambodia performed a Nuremberg-style trial beginning in 2009 with the help of the United Nations to seek justice against one of the greatest crimes of the 20th century by a fanatical Communist regime. Almost 30 years passed before any Khmer Rouge leaders were brought to account for the killings. Finally, two of the regime’s last surviving leaders were imprisoned for life for crimes against humanity.

Survivors felt empowered to tell their true horror stories of their lives under the regime to shocked audiences. Healing began.

The current generation of young people in Cambodia, a country primarily of Buddhists, have no memory of the genocide. Those who personally remember the terrors comprise less than 10% of the population. Two-thirds of Cambodians today are under age 30, growing up decades after the Khmer Rouge’s brutalities and thus having no direct experience of the atrocities.

The country observes an annual National Day of Remembrance every May. In its first year, 1984, it was called The National Day of Hatred Against the Genocidal Pol Pot-Ieng Sary-Khieu Saphan clique and the Sihanouk-Son Sann Reactionary Groups. Many Khmer Rouge members were only teenagers when they committed brutalities against fellow Cambodians, but some remain in power in the government, which controls disseminated information.

Families of persecutors and survivors now live side by side, as the trauma largely recedes into the background of daily life in Cambodia. But the world must remember.