Thet Sambath is a respected Cambodian investigative journalist but Enemies of the People, his documentary on the motives behind the deaths of 2 million Cambodians in the late ‘70s, is not a detective tale.

It’s clear from the outset who the bad guys are, or were: people like Nuon Chea, who was a Khmer Rouge party leader and second in command to Pol Pot. And Pol Pot himself, now dead, the architect of a genocide that killed a quarter of all Cambodians. Men, too, like Khoun and Suon and possibly thousands of other poor Cambodian farmers — intoxicated by the regime’s promises of better living or just swept up in the climate of violence, they became the party’s knife-wielding shock troops.

The trick for Sambath, though, is getting these stories on the record. To him, there haven’t been enough official documents or testimony to explain why the killing happened. Ostensibly political purges of the kind that happened in many Communist states, the killings ended up as an indiscriminate blood bath. One person in the film tells a story of men throwing children in the air and catching them on the tips of long knives.

At the lowest levels, blame for the murders has always been pushed up the command chain. Party leaders, though, have long said they had no idea of the scale of the killings. They attribute them to an excess of murderous zeal at the bottom.

But Sambath isn’t trying to assign guilt — to him, these men are all guilty. Nor is he after revenge — he often assures farmers that the U.N. war crimes tribunal has no interest in ground-level killings. Instead, he wants explanations. He’s searching for anyone who can tell him why these atrocities happened.

It’s a trick that, in the case of Nuon Chea, takes Sambath nearly a decade to pull off. He spends weekend after weekend with the party leader in his country home, discussing the regime as “history,” gradually gaining his trust. He’s eventually able to gain insights and admissions from Nuon Chea that no one else has ever gotten.

Soon after their last interview in 2007, Nuon Chea was arrested by the U.N. on charges of war crimes. This documentary will serve as testimony at his trial.

Making the film costs Sambath’s family, who rarely see their husband and father, and who sometimes go hungry so the man can buy film. But the price the family pays yields a wealth of footage, culled from hundreds of hours of big admissions and small moments.

More affecting than the admissions, though, is the way murder after murder seems to have dulled the emotions of the rank and file. Khoun often expresses regret at the atrocities he committed. We have every reason to believe he is sincere — he spends years following Sambath around to aid the search — but his sincerity carries no strength of feeling.

Suon is the same, at one point demonstrating how he would slit people’s throats in a way that would strangle their cries of pain. Without a hint of emotion, he then says, “[When] my hand started to ache, I switched to stabbing the throat.”