This week, director Martin Scorsese releases his film Killers of the Flower Moon: the true story of the mass murder of Osage Native Americans and the plot to steal the tribe’s oil wealth. The film is a powerful telling of what came to be known as the Reign of Terror, a period that resulted in the deaths of as many as 200 Osage. But the story didn’t end there. For the past 27 years, I have been reporting on what happened afterwards. My documentary Long Knife – produced by George DiCaprio, with his son Leonardo’s encouragement – recounts, in the words of the Osage people, what happened in the century since the killings portrayed in the film, from the Terror to oil thievery to today’s fight for sovereignty. Over the past century, the Osage Nation has continued to suffer massive oil thievery, impoverishment and oil sludge poisoning on their Oklahoma reservation. “It’s not over,” Osage principal chief, Geoffrey Standing Bear, tells me. “It’s still happening.” At the heart of it is legal control of Osage native land by the US Bureau of Indian Affairs, an entity the Osage call the Ma-he-tah, or the Long Knife. Standing Bear, a lawyer himself, likens the arrangement to a military occupation. The Osage’s current misfortunes began in 1894 with, ironically, the discovery of a gigantic oil reservoir under their Oklahoma land. Suddenly, desperately poor Osage became the richest people on Earth. But for the US government, that was too much oil and too much money under the control of a people who were not at that time recognised as US citizens. In 1906, the US Congress passed the Burke Act, named after congressman Charles Burke, who called American Indigenous people “half animal”. Burke would head the Bureau of Indian Affairs, which determined that Osage were not sufficiently competent to handle their new wealth. It assigned each full-blood Osage a white “guardian”. These guardians wrote themselves into Osage wills and insurance policies, then systematically murdered their wards and took their oil rights. That’s where Killers of the Flower Moon ends, in the 1920s. But the predation continued. The US government barred the tribe from developing their own oil and instead auctioned off the Osage’s drilling rights. The giant reserves were then exploited by the behemoths we now know as Getty Oil, ConocoPhillips, Sinclair and Exxon. The Osage were left with small “stripper” wells producing too little oil to send out through pipelines. Beginning in the 1960s, a small operator out of Wichita, Kansas, Koch Industries, agreed to send out small tanker trucks to take the Osage crude. Except that Koch truckers would take 30 barrels and write down 20. In 1996, I was brought in as a forensic expert on energy frauds. I calculated they had skimmed off $2.4bn (£2bn) – about $6bn in today’s money. It’s said that behind every great fortune is a great crime. It was this Osage oil that created one of America’s greatest fortunes: the Koch family, whose wealth is calculated at over $120bn. Kochs have used this wealth to build a fearsome ultra-rightwing force that can create and destroy political careers. Lisa Graves of True North, an authority on corporate lobbying, calculates that Koch interests have spent no less than $200m on campaigns to attack climate change science. To the Osage, it’s still raw. Only months ago, Everett Waller, Osage’s resource chair, confronted the Bureau of Indian Affairs at a tense hearing. “When you get a quote from Koch Oil that said they deserve a barrel for every two they had to pay for, you should have hung the bastards.” (Appropriately, in Killers, Waller plays the fierce Osage leader, Paul Red Eagle.) Today, the Koch oil trucks are gone, but their poisons are left behind – and I’m not just talking about the economic legacy. Former Koch trucker Jack Crossen told me that Koch ordered its workers to cover up toxic sludge spilt into creeks and water supplies. Right now, Chief Standing Bear is at war with the system that engendered the Koch heist, the 1920s Reign of Terror and continuing cruelties. The US government still claims ultimate power over Osage money and lands, simply changing the murderous “guardianship” scheme to the “trusteeship” of the Bureau of Indian Affairs. The chief explains: “This is our land, and we bought this land with our own money. But the federal government says: ‘The ownership title may belong to you, but the day-to-day control and operation belongs to the federal government.’ And who do they get to help manage? Koch oil!” For the chief, it’s also personal. The US government to this day lists Chief Standing Bear as “incompetent”, despite his national recognition as one of America’s top trial attorneys. During those few years of oil wealth 100 years ago, the Osage invested in education and skills, with some becoming noted scholars at Stanford and Oxford, and they gave America its first prima ballerina, Maria Tallchief (née Elizabeth Marie Tall Chief). The chief took me to the Bureau of Indian Affairs office, where, in a huge leatherbound book, he and other college graduates in his family are listed as incompetent. It’s more than an insult. It is part of the legal structure that allows the federal government to remain as sovereign over reservation affairs. Today, Koch’s trucks are gone, but Koch’s campaign to deny climate science and stop the government taking action on greenhouse gas emissions has undermined Osage demands for funds to seal up the thousands of methane-spewing, poisonous wells abandoned by corporate drillers. And Waller needs the sovereign rights granted to other Americans so the Osage can launch his long-term plan to “put it all underground” – that is, instead of drilling oil, drilling huge, clean geothermal reserves. That puts the Osage on the frontline in the war over climate and Koch-ocracy. While the Osage are appreciative of Scorsese and Leonardo DiCaprio bringing the 1920s Terror to the screen, they want the world to know that their story doesn’t end when the movie credits roll. Former chief Jim Roan Gray, whose great-grandfather’s murder is at the centre of Killers, says the Osage want to be seen as more than victims: they are warriors confronting their US rulers for control of their own land and lives. Greg Palast is an investigator of corporate fraud and racketeering turned journalist. His documentary Long Knife: Osage Oil and the New Trail of Tears will be released in early 2024