BOB ABERNETHY, anchor: Of all the atrocities of our times, one of the worst was what happened in Cambodia 25 years ago. The tyrant Pol Pot and his Communist Khmer Rouge forces killed or caused the death of nearly 2 million of their countrymen -- about one person in four. Last week, at long last, the United Nations and the Cambodian government agreed to set up a tribunal to bring to justice the surviving leaders of Pol Pot's reign of terror. Lucky Severson reports from Phnom Penh.

LUCKY SEVERSON: In the middle of Cambodia's capital city of Phnom Penh, there's an old, deserted building that has become the city's most popular attraction. Unlike most tourist places, this one is very dark. Years ago it was a school. But it didn't become notorious until after it was converted into a political prison called S-21. For several years S-21 was a place of indescribable cruelty. They said then that you could hear the screams from blocks away. Today there is only an eerie silence.

It was during Cambodia's brutal civil war when the country's new dictator, Pol Pot, declared 1975 year zero. It was to be the beginning of time, the dawn of a new Marxist utopia.

What most Americans know about Pol Pot and the Khmer Rouge, they learned from the Academy Award-winning movie called THE KILLING FIELDS.

The reality of what happened in Cambodia was much more hideous than what was portrayed in the movie. For the next four years, schools and hospitals were emptied, factories were destroyed. Pol Pot ordered his Khmer Rouge forces to purge the country of all external and internal influences, external being the CIA and KGB. Internal were the Cambodian people themselves, especially those with education.

TOM FAWTHROP (Journalist and Author, GETTING AWAY WITH GENOCIDE): How could a journalist, however seasoned, not feel absolutely schlocked in the stomach?

SEVERSON: Tom Fawthrop is a Cambodia-based journalist and author.

Mr. FAWTHROP: Hard to explain the kind of emotion you feel when you see every fabric of a society, from the culture, their religion, their temples, their monuments -- almost everything had been horribly scarred, defaced, or destroyed.

SEVERSON: Altogether, 17,000 Cambodians were imprisoned at S-21. Seven made it out alive. Many of the dead died horribly. The pictures on the walls tell the story. The Khmer Rouge were proud of their work, according to Youk Chaang, director of the Cambodian Documentation Center.

YOUK CHAANG (Director, Cambodian Documentation Center): They took pictures, it's like they wanted to document the atrocities? No, they want to document the victory, because to them, the photo of a prisoner, or the confession, or the report showing that a prisoner has been killed or eliminated is a sign of victory.

SEVERSON: [During] Pol Pot's reign of terror, his Khmer Rouge brutally murdered nearly two million of their own countrymen. Behind me they found a mass grave that had 166 bodies without heads. This was one of the killing fields, one of many killing fields. Nearly 9,000 Cambodians were executed here, many of them babies, children, moms, and dads.

It's been more than two decades since Pol Pot's regime crumbled. Pol Pot is dead, his body cremated here, on what was a pile of burning tires. Still there has been no trial, no war crimes tribunal, nothing to bring any of the surviving Khmer Rouge leaders responsible to justice. For years, Cambodia and the U.N. have quibbled over details of the tribunal, and then in spring 2001 the United Nations suddenly pulled out of the negotiations.