In the 1920s, oil-rich Osage Indians in Oklahoma kept mysteriously dying. During a period that came to be called the "Reign of Terror," scores, perhaps hundreds, were murdered. And in a nation where Indigenous people faced rampant discrimination, tribal leaders struggled to find answers—or justice. Ultimately, the nascent Federal Bureau of Investigation, under its newly appointed director J. Edgar Hoover, took on the case, uncovering a shocking conspiracy.

One by one, in the early 1920s, Mollie Burkhart’s family members turned up dead. Her sister Anna had been discovered in a ravine in May 1921 with a bullet wound to the back of her head. Following the shooting of a cousin less than two years later, Mollie’s sister Rita and her husband were killed when an explosion reduced their house to kindling. Mollie suspected poison was to blame for the unexplained ailment that killed her mother, and in retrospect, even the “wasting illness” that had killed a third sister, Minnie, in 1918 seemed suspicious.

It wasn’t just Mollie’s family that was being methodically killed on Oklahoma’s Osage Nation Reservation in that decade. More than two dozen members of the Osage tribe had been shot, stabbed, beaten and bombed in one of the bloodiest crime sprees in American history.

Investigators who probed the case too deeply also had a propensity for turning up dead. One attorney with information on the case was thrown off a speeding train, while the body of Barney McBride, a wealthy white oilman who agreed to go to Washington, D.C., to ask federal authorities to investigate the murders, was found stripped, beaten and stabbed more than 20 times in a Maryland culvert in what The Washington Post called “the most brutal in crime annals in the District.”

As journalist and author David Grann details in his 2017 book, Killers of the Flower Moon: The Osage Murders and the Birth of the FBI, adapted into a 2023 film by director Martin Scorsese, the Osage reservation was soaked in blood because it was awash in oil. Driven from their lands in Kansas, the Osage had bought a swath of northeast Oklahoma in the early 1870s. The rocky, barren reservation promised to yield little—with the exception of their desire to be left alone—until the discovery of one of the largest oil deposits in the United States below the surface.

The Osage had shrewdly retained the rights to any mineral discoveries, and oil barons such as J. Paul Getty, Harry Sinclair and Frank Phillips paid grand sums for leases at outdoor auctions held under the boughs of a vast tree dubbed the “Million Dollar Elm.” Each member of the Osage tribe received quarterly royalty payments for their "head rights," and as the years progressed, so did the number of digits on their checks, growing into the hundreds and then the thousands of dollars. In 1923 alone, the 2,000 tribe members collectively received $30 million—the equivalent to $400 million today, according to Grann.

The Osage became the richest people per capita in the world. “They lived in mansions and had chauffeured cars. They had servants, many of whom were white. These images belie long-standing stereotypes of Native Americans that trace back to the first contact with whites,” Grann tells HISTORY. “It flips our conventional thoughts on their heads.”

Even the Osage’s blessings turned out to be cursed, however. The great wealth lured not only desperadoes, bootleggers and criminals—but fantastic jealousy as well. “The Osage Indians are becoming so rich that something will have to be done about it,” reported Harper’s Monthly.

“Prejudice provoked a scapegoating of the Osage for their wealth, and the U.S. Congress literally holds hearings about what the country could do in response,” Grann says. Lawmakers appointed local white guardians to approve every expenditure by the full-blooded Osage “down to the toothpaste they purchased at the corner store,” Grann writes. “It’s a system rooted in racism, done under the pretense of enlightenment that the Osage needed protection,” Grann says. “Even worse, it led to an entire criminal enterprise that had been sanctioned by the U.S. government.”

Swindling the very people they were assigned to protect, guardians forced the Osage to purchase goods from them at inflated prices and received kickbacks by directing them to do business with certain stores and banks. In some cases, guardians dropped any pretenses and simply stole the money—at least $8 million, according to one government study. “They’re scalping our souls out here,” complained one exasperated Osage. The systematic embezzlement—referred to as the “Indian business” by some white settlers on the Osage reservation—wasn’t lucrative enough for some, however.

In order to maintain tribal control, shares of the oil money could not be sold by the Osage to white settlers, but they could be inherited. That loophole proved the genesis of a calculated, cold-blooded plot to gain inheritance rights from tribe members before killing them. In some instances, white settlers even married their marks to legally become the next of kin before murdering their spouses.

As the body count rose in the early 1920s, the Osage saw no action from local and state law enforcement personnel. “There was a tremendous amount of corruption in Osage County. The power structure was able to buy off lawmen. In some cases lawmen were directly complicit or turned a blind eye,” Grann says.

The tribe sent representatives to Washington, where they appealed for help from President Calvin Coolidge—and then directly from the relatively new Bureau of Investigation (which would be renamed the Federal Bureau of Investigation in 1935). Seeking a high-profile success to erase the bureau’s stained reputation for its role in another oil corruption scheme, the Teapot Dome scandal, newly appointed director J. Edgar Hoover sent investigators to Oklahoma.

“What’s interesting is you get to see Hoover and the bureau in its formative period. You see all the seeds of his character—paranoia, ambition and manipulation. He’s very insecure in his career at the time,” Grann says. “The bureau badly bungled the case initially. They released an outlaw named Blackie Thompson, hoping he would work as an undercover informant, but he instead robbed banks and killed a police officer. At one point Hoover wanted to get out of it and turn it back to the state, but after the scandal he didn’t have a choice.”

Hoover instead turned the case over to Tom White, an experienced investigator who lived in the saddle. The former Texas Ranger put together an undercover team that included a Native American agent. “They worked quickly and methodically. White pursued the case when many people believed the people they were pursuing were untouchable because they were white and the victims were Native Americans,” Grann says.

White was able to crack the murders of Mollie Burkhart’s family, uncovering an insidious conspiracy by a prominent local cattle baron, William Hale. While positioning himself as a benevolent community leader and generous friend to the Indians, Hale encouraged his nephew Ernest Burkhart to marry Molly, then methodically arranged to have her kin murdered (by shooting, poison and bombing) so that Burkhart and Hale would inherit the valuable oil rights.

But beyond the Burkhart case, which ultimately garnered national attention, many of the Osage killings remained unsolved. “Hoover was in a rush to close the case—really the case was closed prematurely,” Grann says. “The bureau didn’t reveal a deeper, darker conspiracy, and as a result many were able to escape justice.”

In Killers of the Flower Moon, Grann picks up the case and reveals the even wider conspiracy in the Osage murders, which may have numbered in the hundreds. “There really was a culture of killing and a culture of complicity. That’s what makes these crimes so sinister and disturbing,” he says. “This really was about a clash of two civilizations, the emergence of modern law enforcement and how important it is to be a country of laws.

Unlike state and local investigators, Hoover’s agents provided the Osage with some relief from their “Reign of Terror”—but also a bill for more than $20,000 for their services. “That’s one of the more outrageous details,” Grann says. “They had to pay for justice.”