It’s not often that an invitation to the Oscars gets turned down. Most of those who were nominated last week will enjoy the additional attention, interviews and the customary red-carpet wardrobe decisions. But for the anonymous co-director of The Act of Killing, which was nominated for Best Documentary Feature, publicity is not an option.

For the Indonesian crew of more than 60 people, public accolades went out the window when they decided to remove their names from the film’s credits due to fear of reprisals from anti-communist militias.

“It is not possible for me to go to the Oscars, it is too much publicity and it is not really safe for me to be openly seen as the co-director of the film,” the Jakarta-based director told The Independent. “I’m not the kind of person that likes to brag about my achievement… anonymity suits my personality.”

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The documentary, directed by Joshua Oppenheimer and executive produced by Errol Morris and Werner Herzog, has been widely praised for its portrayal of the men responsible for the mass killings that followed a failed coup attempt by a group calling itself the September 30 Movement in Indonesia in 1965.

That rebellion was quelled by General Suharto, who seized power from President Sukarno, and, in what became known as the Communist Purges, set about a bloody retribution on the alleged communists, ethnic Chinese and left-wing intellectuals he blamed for the coup attempt. Between the security forces, local militias and vigilantes they killed more than one million people within a year.

The film enlists the ageing, real-life mass killers – a group of small-time gangsters at the time who were hired by the military – to re-enact their killing sprees in a sequence of surreal movie parodies, revealing the untold horrors airbrushed out of Indonesian history.

The Act of the Killing reveals not just the impunity enjoyed by the killers today, but the celebrity status they have in parts of Indonesian society. Gang leader Anwar Congo, personally responsible for more than a thousand deaths, is hailed on national television for his efficient killing methods; in another scene he casually talks about which clothes are best suited for a massacre.

“Often films or books investigating mass murderers focused on those who lost and subsequently were told by society that they were wrong, and the killers tended to express remorse,” says Andreas Harsono, Indonesia researcher for Human Rights Watch. “The Act of Killing, in contrast, shows the emotions that the killers had at the act of killing – they really enjoyed it – and for that reason it is very disturbing.”

The Communist Purges have never been publicly discussed in Indonesia. In schools children learn that the killings were the necessary work of patriots, resulting in less than 80,000 deaths. Academics say that Indonesians still view the Communist Party of Indonesia (PKI) as “fundamentally evil” and describe the killings as a matter of “them or us”.

“Although it is sometimes said that the killings were used as a time to settle old scores, and although survivors sometimes claim that they were targeted for non-political reasons, we have very little evidence of people who were not on the left being targeted,” says Professor Robert Cribb, an expert on the Communist Purges from Australian National University. “Those who took part in the killings were widely seen as having done Indonesia a service.”

Oppenheimer chose to bypass government censors to avoid having his film banned in Indonesia, depending instead on a virtual campaign – helped by Indonesians on social media – and word of mouth via private screenings.

Paramilitary organisations still have strong influence in parts of Indonesia, meaning anyone associated with The Act of Killing faced unknown risks. Two screening organisers received death threats and last year the manager of a local newspaper was beaten for writing an article promoting the film.

Screenings have been held in 120 cities across the country, while 11,000 people have downloaded the film since it was made free to Indonesians in November. One request for a private screening involved taking a DVD to eight different villages in Central Java, all with mass graves near by.

“It changed the way villagers see the victims and their family members,” says the co-director. “They learned that it was not because of a mistake, not because their family did something wrong, but because the whole system is rotten.

“People have called and written to us to tell us how they have been touched by the film, people have started reunions and gatherings, people who are in the same boat together – families of victims gathering with the families of the perpetrators – they want to learn about the mistakes of their parents and their grandparents.”

With a presidential election rapidly approaching in Indonesia the government has refused to engage with the film’s revelations, despite some of the back-slapping cameos they play in the film. No public statements have been made, although one embassy worker in the US divulged to the entertainment website TMZ that they were hoping it stopped winning awards as it was trashing Indonesia’s image, painting it as backward and lawless.

Unfortunately, the bombshell it drops on Indonesia has prevented it from reaching a much bigger audience in neighbouring countries as Asean (Association of Southeast Asian Nations) members tend to give politically sensitive issues a wide berth.

“Distributors in South-east Asia did not think the film would pass censors… Asean countries with close ties to Indonesia do not want to anger their counterpart or encourage their own citizens to dig deeper into their own histories,” says Lorna Tee, a Hong Kong-based film producer.

The co-director spent eight years working on The Act of Killing, knowing from the beginning that it was possible he wouldn’t be credited for the work he did.

“We thought about what should be written – our real names? A pseudonym? – and we decided it best to remain anonymous because actually we didn’t know what would happen,” he says.

Oppenheimer says he left his team, some of whom endured insomnia and nightmares during the filming, to decide how and with whom they discussed their participation, but most have kept it a carefully guarded secret – a few family members at most.