“They won’t remember”, Robert De Niro’s character says in Martin Scorsese’s Killers of the Flower Moon. As the real-life William Hale, a cattle baron in 1920s Oklahoma, he is deluded in thinking that the Osage Nation would move past the memory that dozens of their members who had become rich from oil rights were systematically killed for their money. But the line leaps out of the film as a reminder that much of the world did forget, until the events were restored to the mainstream in David Grann’s dynamic, deeply researched 2017 bestseller, Killers of the Flower Moon: The Osage Murders and the Birth of the FBI, which inspired the film. While he was writing the book and long after, Grann tells BBC Culture, “The most common comment I have received is: ‘I can’t believe I never learned about this'”, adding, “I think that is a reflection to some degree of the underlying force that led to these crimes, which was prejudice.”

Spoiler alert: This article contains plot spoilers for Killers of the Flower Moon.

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Those murders and their near-erasure from history go to the heart of US culture. “American democracy arose from the dispossession of American Indians”, the Yale University historian Ned Blackhawk writes in his recent book The Rediscovery of America: Native Peoples and the Unmaking of US History. “Scholars have recently come to view African American slavery as central to the making of America, but few have seen Native Americans in a similar light,” he writes. His work is part of a trend aiming to restore those injustices to their crucial place.

At the centre of Killers of the Flower Moon, both the book and film, is Mollie Burkhart (Lily Gladstone) a modest, oil-rich Osage whose family was especially targeted, one sister shot to death, another’s house firebombed, their mother most likely poisoned. Later Mollie herself becomes mysteriously, gravely ill. Her true story offers a dramatic example of the cultural atmosphere that allowed what came to be called the “Reign of Terror” to happen, and then to be swept aside.

The pattern of dispossession that began with Columbus continued for hundreds of years. In the 19th Century, the US government forced the Osage off their land in Kansas, so they moved to Oklahoma, where in the 20th Century oil made them fabulously rich, for a time the world’s richest people per capita. Even then, the US government labelled many Indigenous people “incompetent”, a designation that often depended simply on how much Native blood they had. Guardians, often corrupt, were put in place to oversee and restrict how those designated spent their own money. Early in Scorsese’s film we see Mollie meeting with a guardian, even though she is extremely intelligent and capable.

Like the book, the film is extraordinary in the way it captures both Mollie’s intense personal story and the cultural prejudice that fostered the crimes. (Unlike the book, which unfolds like a detective story, the film reveals the killers’ identities early, and there are spoilers ahead for both accounts.) In the early 1920s, William Hale was a powerful force in Osage County, a venal man who regarded Native Americans as less than human while pretending to be the tribe’s friend and benefactor. Hale encourages his equally greedy nephew, the World War One veteran Ernest Burkhart (Leonardo DiCaprio) to marry Mollie. We are deliberately left guessing whether Ernest genuinely loves her, wants to marry for money, or some combination of the two, a question that slowly comes to haunt Mollie as well.

Scorsese’s film, using mock archival newsreels, depicts the baffled resentment with which white society regarded the wealthy Osage being driven by chauffeurs, and Native women dressed in fashionable furs and jewels. Mollie herself wore a traditional blanket as a coat and lived a simple life even though she had a large house and servants. But Grann’s book cites a 1920 Harper’s Monthly Magazine article called “Lo, the Rich Indian!” which refers to the oddity of “red millionaires”. Another typical article in a 1922 travel magazine was called “Our Plutocratic Osage Indians”.

Increased public awareness

Despite the condescension, many white people married into the Osage tribe, with sinister motives. When the US government allotted parcels of Oklahoma land to the Osage, the tribe members kept the rights to profit from the oil, called headrights, which importantly could only be inherited, not sold. Marrying an Osage for the inheritance was a way white people could get their hands on the oil money. As Grann tells BBC Culture, “There was a particular diabolical nature to these [murder] plots because they involve people marrying into families pretending to love you while simultaneously plotting to kill you.” While planning murders in the film, Hale bluntly voices the opinion that the Osage are not worthy of their money and eventually not worthy of their lives.

Hale was not alone. “The more I dug into it, the more I realised this was really about a culture of killing and a culture of complicity,” Grann says. “I found evidence of doctors who were administering poison. I found evidence of morticians who were covering up bullet wounds. Some of the guardians and the lawmen and the prosecutors were on the take and either not investigating these crimes or sometimes maybe even had a hand in them, and many others were complicit in their silence.”

The film reflects that real-life culture of white supremacy. There is a glimpse of the nearby Tulsa Race Massacre in 1921, contemporaneous with the Osage murders, in which a prosperous black neighbourhood called Black Wall Street was burned to the ground by a white mob. The film depicts a parade on the main street of the Burkharts’ town, with robed and hooded Ku Klux Klan members marching in the parade just behind a group of women carrying a banner reading “Indian Mothers of Veterans”, a clear indication of how blatant and entrenched in society white supremacy was.

As the Osage murders accumulated, the crimes became so alarming and so frequent that the FBI investigated. Soon the story was receiving national attention. When Hale was convicted of a single murder, it was reported in The New York Times, but the headline was telling in the way it focused on the white criminals and not their Native victim: ‘King of Osage Hills’ Guilty of Murder: Hale, Cattleman, and a Cowboy Are Convicted of Killing an Indian.

Why did such a volatile story, skewed though it was, then virtually disappear? Tara Damron, program director at the White Hair Memorial, a repository of Osage history, and a member of the Osage Nation herself, tells BBC Culture that the erasure “goes to the overall treatment of Native Americans, and indigenous history not being taught, not being included and those voices being silenced”. Indigenous tribes, she says, “have a government to government relationship with the United States that goes back to treaties. What happened to the Osage People during this terrible time in our history is American history, and this story needs to be told.”

Killers of the Flower Moon in both forms can now be seen as part of a larger cultural reclamation. In 2011, for $380 million, the US government settled a 12-year-long lawsuit the Osage Nation brought against it for mismanaging the tribe’s funds. In announcing it, a statement from the Department of the Interior said the settlement signalled “President Obama’s commitment to reconciliation and empowerment for American Indian nations”. Today’s Secretary of the Interior, Deb Haaland, is the first ever Native American cabinet secretary, another sign of change.

But some injustices can never be redressed. Grann’s research uncovered many Osage murders that were not investigated and can likely never be solved. “Many killers went free,” Grann says, because the FBI “did not actually uncover this much deeper and darker conspiracy that existed.” The witnesses are now dead and the crimes were often not recorded. “Often you just can’t find the evidentiary material to even identify who the perpetrator was and resolve these cases”, Grann says. As the film and book remind us, though, they can be brought to light and remembered. Since the book was published, Damron has found increased public awareness of the murders. She says, “I hope that cultural attitudes have changed but the impact is yet to be seen”.

Killers of the Flower Moon is released on 20 October.

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