Even in his prayers, he has not yet told the souls of his parents, on whose behalf he has been searching.

“One day, I plan to have a ceremony for them,” he said. “I will light a candle and incense, and I will say, ‘Yes, I got success. I understand what happened to you and to other Cambodian people. You can find peace and be reborn into a better life.’ ”

IT was this mission that drew Mr. Thet Sambath into journalism, where he hoped to learn the tools of investigation, he said. He worked first for The Cambodia Daily and now works for The Phnom Penh Post; they are Cambodia’s two English-language newspapers.

“Journalism showed me how to connect with people and how to ask questions,” he said. His years of interviews have made “Enemies of the People,” an engrossing documentary that has been honored at the Sundance Film Festival and at other festivals.

In addition to Mr. Nuon Chea, he connected with low-level Khmer Rouge operatives, some of the people who carried out the executions that were part of Mr. Nuon Chea’s vision.

The results are chilling. In one interview on camera, a farmer describes his work in the killing fields, work in which his hand got so tired slicing throats that he had to change his grip on his knife.

“She was in the last batch,” the killer says, recalling one execution. “We’d killed a lot of them except her. She grabbed my legs, screaming, ‘Uncle, please let me live with you.’ I said, ‘You can’t live with me.’ She said, ‘Please, just let me live with you.’ I said, ‘Will you live with me forever?’ She said, ‘Yes, I will.’ She hugged my knees. Then Eng shouted at me, ‘What are you waiting for? Hurry up and kill her.’