Between 1975 and 1979, the Khmer Rouge dominated Cambodia with a reign of terror that left over two million dead. In "security bureau" S21, around 17,000 prisoners were detained, interrogated, tortured and eventually executed in one of the worst incidents of genocide of the 20th century. In K21: The Khmer Rouge Killing Machine, filmmaker Rithny Panh returns to the detention centre with one of the three men who survived and the guards who were in charge to piece together what prompted such mass murder.

Charting Cambodia's dark years of terror, this sparse documentary delivers a harrowing history lesson. Keeping obsessive records about each new inmate and the torture they received before being killed, the regime built up an archive that details a carefully documented and meticulously executed genocide. This wasn't a secret programme, but a ruthlessly efficient killing machine that forced prisoners to make up bogus confessions before executing them.

"GUARDS JUSTIFY THEIR ACTIONS"

Although offering little historical background, what makes Panh's film so distressing is the way in which the guards justify their actions. No older than teenagers at the time, they claim to have been motivated by a combination of indoctrination and terror. Yet as they re-enact their roles for the camera with an enthusiastic fervour - threatening imaginary prisoners with beatings and torture - it's hard to accept their belief that they were also victims of the regime.

"When I think about it, I get a headache," remarks one of them when quizzed about the blood on his hands. Are they evil, indifferent, or simply unable to acknowledge the extent of their crimes? It's impossible to tell but as Nath - one of the camp's three survivors - points out, no one who was involved in the terror has ever publicly asked for forgiveness or even acknowledged their guilt.

A chilling reminder of man's capacity for brutality, S21 is a disturbing film that ends without offering insight, understanding or cathartic release.