On April 17, 1975, the Khmer Rouge — unsmiling 15-year-olds with AK-47s and orders not to touch anyone — entered Phnom Penh. With their rifle barrels, they prodded the urban population out into the countryside, more often than not to perish in the rice fields. Over the next four years, the Khmer Rouge executed 1.7 million of their own people, a third of all Cambodians.

Who was responsible for this slaughter? The UN-backed trial now taking place in Phnom Penh has charged five Khmer Rouge leaders with war crimes and crimes against humanity; but one by one they die of senility, or are released due to Alzheimer’s, unable to recall what they did. The tribunal’s Cambodian president is forgiving. “These events took place more than 30 years ago and it’s very difficult to remember them.” The tribunal has sentenced just one person: a former teacher called “Duch”, who, as head of security in Democratic Kampuchea (as Cambodia became known in those hellish years), was in charge of Phnom Penh’s main prison, S-21 — in which 12,380 people were methodically tortured. Afterwards, they were put to death in the “killing field” of Choeung Ek, 16 kilometres southeast of the city.

Duch is an unrepentant man with a “full-throated” laugh. Based on hundreds of hours of interviews with him in his comfortable Phnom Penh cell, this remarkable book is by a Cambodian who lived through the Khmer Rouge period. The author’s wrenching account of what he and his family had to endure is mingled with a patient attempt to humanise a person who dehumanised so many. Written in the plain, unhysterical style of Gitta Sereny or Claude Lanzmann, “The Elimination” does not seek judgment, still less revenge. “What I’m looking for,” says Rithy Panh, “is comprehension.”

Panh was 13 when he was forced to join the mass exodus. His father, who later starved himself to death, was a peasant who had risen to be chief undersecretary of education — but education counted for nothing now. Duch told Panh: “We were going to be done with the bourgeois and capitalist classes ... also students, professors, doctors, engineers ... All were sent to the provinces to do productive work.” Schools were closed and turned into torture centres — including the lycee I attended in the Sixties (along with several pupils who became Khmer Rouge leaders). Books, newspapers and spectacles were banned.

In Duch’s classless new world, the words “study” and “reeducation” carried different, fatal meanings. A Khmer Rouge commander would drop by one evening and point to several persons. “The Angkar [the Organisation] has chosen you. You’re to be sent away to study. We leave at once.” The following day, their bodies would be found with their heads smashed, unburied, eventually submerging with the rains. “They fertilised fear,” writes Panh — who risked his life when he was caught reading the French label on a medicine box. Reading was forbidden, like marrying for love, or wearing coloured clothes, or praying.

In January 1979, the Vietnamese army “liberated” Phnom Penh. Twenty years later, Duch was tracked down by the British photographer Nic Dunlop, and arrested. He had exchanged one ideology for another, and converted to Christianity. “He was baptised in a river, but under a false name,” observes Panh, who wants to give the laughing executioner a chance to explain himself. But Duch is slippery. His words slide “as though on runners”. He takes refuge in slogans, in denials and omissions. “I assume entire responsibility for S-21,” — and in the next breath: “I never tortured anybody.” He maintains: “I was always poring over my files. I didn’t hear my child crying.” But nor did the conscientious Duch, apparently, hear the cries of his own schoolteacher being raped with a piece of wood; nor the screams of her husband who was forced to eat his excrement and afterwards was electrocuted — to gain a confession everyone knew to be fictitious, that he worked for the KGB/CIA/the Vietnamese. Nor did Duch hear the moans of the wife of Lon Nol’s minister of education, who was dissected alive and had her blood drained into four bags (“total bloodtaking” was Duch’s phrase).

Further, Duch claims that he never saw blood, barbed wire, the executions at Choeung Ek, or the hurling of children from a third-floor balcony before their parents’ eyes, after Duch had written down his orders for the day (on one of thousands of pages which he abandoned in a panic): “Grind them into dust.” In another life, Duch, an admirer of Gandhi and Leonardo, says, “I’d like to have been Pierre Curie.”

Rithy Panh is a filmmaker, and his method of entrapment is montage. Bit by bit, Duch’s mendacity is revealed. The figure that emerges is all too human, who wets himself when on the telephone to his boss; who drinks coconut milk laced with Cointreau; who calls a woman he fancies “my frangipani flower”. Against the confusion and, increasingly, the silence that characterises our response to whatever happened in Cambodia between 1975-79, Panh has built a monument worthy of Primo Levi. “It’s a struggle against elimination,” he writes of his project, which succeeds in showing that Duch and his ilk cannot be reduced to a geographical peculiarity.

In this book, and in his films, Panh has pincered the truth behind their mass crimes more effectively than any trial. “Like it or not, the history of Cambodia is in the deepest sense our history, human history.”

The Elimination: A Survivor of the Khmer Rouge Confronts His Past

By Rithy Panh with Christopher Bataille,

Clerkenwell, 320 pages, £12.99