Month after month, he said, he sat for hours with the aging leader, Nuon Chea, sharing meals and confidences, recording his words on thousands of hours of audio and videotape until at last he confessed his guilt.
His pursuit of Mr. Nuon Chea became an obsession that he said he hid from everybody, even his wife, who never knew where he went on what he called his investigation.
“I forgot everything,” said Mr. Thet Sambath, 42. “I forgot how to make money for my family. I sold my land. I sold everything. My brother told me, ‘You should stop going to the province. For what? You should take care of your children, your wife, build a house.’
“He didn’t know what I was doing. I never told anybody. If I told them, they would have told me to stop.”
Mr. Nuon Chea, the chief ideologue for Pol Pot, who died in 1998, is one of four Khmer Rouge leaders who are due to be tried next year for crimes against humanity in the deaths of 1.7 million people from 1975 to 1979. That trial follows the conviction last month of the chief Khmer Rouge prison warden, Kaing Guek Eav, known as Duch.
The killings were a necessary part of the revolution, Mr. Nuon Chea told Mr. Thet Sambath during their discussions, which form part of “Enemies of the People,” a film about Mr. Thet Sambath’s search for answers that is now being screened in New York and Los Angeles. Mr. Thet Sambath and his British co-producer, Rob Lemkin, have refused a request by the court for a copy of the film, saying they promised Mr. Nuon Chea that his remarks would be used “for history, not for evidence.”
“They were killed and destroyed,” said Mr. Nuon Chea, now 84 years old, fragile and ailing but unrepentant. “If we had left them alive, the party line would have been hijacked.”