In his genre-bending documentary, "The Act of Killing," director Joshua Oppenheimer invites a group of former Indonesian death-squad leaders to reenact the real life executions they performed in the 1960s. (Lee Powell/The Washington Post)

To describe “The Act of Killing” as a riveting documentary about Indonesian death squads that terrorized that country’s citizens in the 1960s might be factually accurate. But it doesn’t get nearly to the heart of it. This audacious, horrifying, boldly experimental plunge into the mind-set of murderers and the culture of impunity breaks so many rules of documentary decorum that it virtually creates its own genre: investigative improv, perhaps. Or, better yet, Brechtian nonfiction.

Whatever you call it, “The Act of Killing” is a must-see. Using blunt stagecraft, probing psychological insight, elegant interrogation of narrative truth and characters steeped in a particularly terrifying brand of self-mythologizing, director Joshua Oppenheimer has succeeded in turning “The Act of Killing” into both a sharply confrontational vehicle for bearing witness and a craftily layered meditation on the cinematic medium itself.

“The Act of Killing” focuses on the years 1965 and ’66, when a young man named Anwar Congo became a legendary death- squad leader in North Sumatra, murdering communists, ethnic Chinese and intellectuals following the military coup that brought longtime authoritarian leader Suharto to power. It’s estimated that 1 million people died during those purges, which were carried out with the help of a paramilitary organization called the Pancasila Youth, an organization that thrives even 15 years after Suharto’s resignation.

Despite its historical context, “The Act of Killing” contains none of the expected stock footage or newsreels of atrocities and trials. Rather, Oppenheimer catches up with Congo, his cronies and Pancasila Youth leaders in the present day, as they proudly recall their actions 50 years ago. What becomes unnervingly clear as Oppenheimer films these men swaggering through city streets, shaking down shopkeepers and bullying citizens who nervously laugh along with them, is that not only have they not been prosecuted for their crimes, but they’re also lionized for them. Congo is so proud of his past deeds that he eagerly shows Oppenheimer how he preferred to dispatch his victims, strangling them with a taut piece of wire so they would bleed less.

As “The Act of Killing” progresses, Congo and his fellow criminals explain that they learned most of their postures and methods from Hollywood crime movies, for which they were scalping tickets when they were enlisted to become freelance domestic terrorists in the 1960s. Citing Marlon Brando and Al Pacino as role models, they call themselves “gangsters” throughout the film, reminding anyone who will listen that the word for “gangster” in Indonesian has its roots in the term for “free man” in Dutch. In time, Congo and his colleagues are donning garish costumes and bloody makeup to reenact the torture and murders they committed, staged like cheap film noir knockoffs with chillingly bad dialogue.

It’s a risky proposition to ask such morally challenged protagonists to reenact their crimes as pulp entertainment. And in one anguishing scene of a scene, a survivor confronts his stepfather’s killers with raw, tearful emotion, eliciting shockingly dispassionate responses. Thankfully, Oppenheimer — who has made earlier documentaries focused on human rights issues — has the brains and taste to make those sequences revealing rather than exploitative. It’s telling that his executive producers on the project are Errol Morris and Werner Herzog, both masters of allowing their subjects’ self-deception to propel alternately sobering and gobsmacking stories.

“The Act of Killing” records such perversity, such venal cruelty and vile narcissism, that it’s often difficult not to shudder and turn away. And those moments of revulsion are just as likely to occur during a staid round of golf or in the gilded halls of government as in the film’s more lurid set pieces. With its subtle connections of screen violence and real-life violence, “free men” and free markets, and the ongoing cultural legacy of co­lo­ni­al­ism, “The Act of Killing” is a brilliant, powerful reckoning with the wages of history, mendacity and denial.

Killing is always an act, as the film suggests — physical, theatrical and ethical. And its own fascinatingly ambiguous final sequence leads viewers to question when repentance can be one, too.

★★★★

Unrated. At Landmark’s E Street Cinema. Contains violence, profanity, drug references, smoking and disturbing material. In Indonesian and English with subtitles. 122 minutes.