To say that Joshua Oppenheimer's The Act of Killing was lauded upon its 2012 release would be a considerable understatement. There were, of course, awards and high placings on most critic's lists. But no amount of plaudits seemed adequate for such an extraordinary document, in which ageing former paramilitaries gleefully re-enacted killings from the Indonesian genocide of 1965 and 1966.

Oppenheimer's follow-up, though not nearly so high concept, makes for equally discombobulating viewing. The Look of Silence follows a young optometrist, Adi Rukun, as he searches out the men who killed his brother, Ramli. In 1965, two men – Amir Hasan and Inong – captured Ramli at his parents' home in North Sumatra. They dragged him to the spot where they killed some 10,500 others, mutilated him with machetes, cut off his penis and dumped his body in a river.

"There's a scene in The Look of Silence that's really the inspiration for both films," says Oppenheimer. "It's the two men going down to the river and taking turns being victim and perpetrator. And they pretend to be proud – and I'd emphasise the word pretend – of what they had done. That was the first day I had brought perpetrators together and I was wondering if they would boast for each other in the same way they'd boast to me. When I saw they were even worse when they were together – as if reading from a shared script – I could no longer pretend these men were psychotic. I had to acknowledge the systemic nature. This was political psychosis."

Born in Texas and raised between Washington DC and New Mexico, Oppenheimer has spent more than 12 years on his Indonesian diptych. His engagement with the subject has roots in his politically conscious childhood: his father was a political science professor, his mother was a trade union activist and his paternal grandparents fled Germany just before the Holocaust.

“That day by the river I had this awful feeling as though I had wandered into Germany 40 years after the Holocaust only to find that the Nazis were still in power with support from the rest of the world,” says the filmmaker. “And moreover, my understanding of Cold War-era atrocities committed across the global south, led me to reason this is not a freak occurrence. So I decided to drop everything else I was working on to focus on this entirely. There could hardly be anything more important. This impunity is the story of our time.”

Drinking blood

More than a million people were killed by mostly amateur assassins in Indonesia in 1965. The assassins had been ordered by the army to slaughter at will to consolidate the rule of General Suharto. In The Look of Silence various perpetrators recall cutting off breasts and drinking their victims' blood. A daily dose of human blood was recommended to counteract any ill effects from taking part in mass murder.

"The perpetrators in Indonesia differ from other perpetrators in that they've never had to apologise for what they've done," says Oppenheimer, who remains in touch with Anwar Congo, the killer at the centre of The Act of Killing. "They've never been asked to feel ashamed. I think all of the men I filmed are haunted by unspeakable memories. But we're dealing with people who are still in power and who believe the rest of the world celebrate what they've done. They still have a victors' history available to celebrate what they've done."

Oppenheimer does not regard either work as a campaigning film but The Act of Killing and The Look of Silence have had an impact in Indonesia.

"The Act of Killing had a wide release in Indonesia and then we made it available for free," says the filmmaker. "It has been downloaded millions of times. When it was nominated for an Academy Award a spokesman for the president acknowledged finally that they knew what happened in 1965 was a crime against humanity and there was a need for reconciliation but they'd do it 'in our own time'. But it was important, because it was the first time they acknowledged what happened."

Death threats

The Look of Silence has been distributed across Indonesia by two government agencies and the National Human Rights Commission. Many Indonesians have hailed the quiet, dignified protagonist as a hero. And yet, it's striking how many of the final credits simply read "Anonymous".

“Adi is now at the centre of the movement for truth and reconciliation,” says Oppenheimer, who himself continues to receive death threats. “But to ensure his safety took the work of some 20 people to relocate the family, to get the children into better schools, and to help raise funds for Adi to open an optometrist’s shop.

“The family are doing well but they are miles from where they come from. They’ve had to become fugitives for their attempt at reconciliation. It’s a sign of how far Indonesia has to go towards genuine democracy and the rule of law for everybody.”