The Osage Reign of Terror was a period in the 1920s where dozens of Osage tribe members were murdered. The motive was the oil resting under their land that they leased to drilling operations for millions of dollars. According to David Grann, the author,

The official death toll of the Osage Reign of Terror had climbed to at least twenty-four members of the tribe. Among the victims were two more men who had tried to assist the investigation: one, a prominent Osage rancher, plunged down a flight of stairs after being drugged; the other was gunned down in Oklahoma City on his way to brief state officials about the case.

The body count and violence of the case drew national attention and the FBI was sent to investigate. It wasn't the modern FBI, however, and the Osage case was one of the first that caused it to get national attention.

A lot of the narrative focuses on Mollie because her husband is connected to Osage murders. She sees family members die and then she becomes ill enough that her four-year-old daughter, Anna, has to go stay with another family. Grann writes that "in late 1925, the local priest received a secret message from Mollie. Her life, she said, was in danger. An agent from the Office of Indian Affairs soon picked up another report: Mollie wasn’t dying of diabetes at all; she, too, was being poisoned." A number of Osage had died from unexplained illnesses and Mollie was now also clearly a target.

Tom White is the FBI agent put in charge of the Osage case. Grann says,

When White entered the bureau, it still had only a few hundred agents and only a smattering of field offices. Its jurisdiction over crimes was limited, and agents handled a hodgepodge of cases: they investigated antitrust and banking violations; the interstate shipment of stolen cars, contraceptives, prizefighting films, and smutty books; escapes by federal prisoners; and crimes committed on Indian reservations.

However, the next decade transformed the organization. They became more instrumental in solving crimes of all sorts all across the country. Part of this was due to the national attention the FBI got from the Osage murders.

White discovers that there is a way for someone to gain access to the land. He looks at the pattern of murders and sees that the land—and the millions it produces—could be transferred from the tribe through inheritance. This is what leads investigators to the culprit, William Hale.

There was one legal way, though, that someone could still obtain a headright: inheritance. As White examined probate records for many of the murder victims, it was evident that with each successive death more and more headrights were being directed into the hands of one person—Mollie Burkhart. And it just so happened that she was married to Hale’s nephew Ernest, a man who, as an agent wrote in a report, "is absolutely controlled by Hale." Kelsie Morrison, the bootlegger and bureau informant, said to agents that both Ernest and Bryan Burkhart did exactly what their uncle told them to do. Morrison added that Hale was "capable of anything."

During the subsequent trials, it's shown that Hale is a master of manipulation. He manipulates the media, the witnesses, and the system itself to try to get out of trouble. Ultimately, however, he does have to go to jail.

Grann isn't completely satisfied with the outcome of the trial. He believes that more people were involved and that it was too large and far-reaching to have been the work of one man. He writes:

The authorities insisted that once Hale and his conspirators were given life sentences, they’d found the guilty parties. And after White had taken the job at Leavenworth, the cases were closed, closed with great triumph, even though the bureau had not yet connected Hale to all twenty-four murders. Was he really responsible for every one of them? Who, for example, had abducted the oilman McBride in Washington, D.C., or thrown W. W. Vaughan off the speeding train?

Grann researches the case until he finds evidence that more people were involved. During the years, multiple Osage Indians were murdered in the guardianship of other people. It appears that some were poisoned. For example, he says:

I searched for the name of H. G. Burt, the suspect in W. W. Vaughan’s killing. The log showed that he was the guardian of George Bigheart’s daughter as well as of four other Osage. Beside the name of one of these wards was the word "dead." I then looked up Scott Mathis, the owner of the Big Hill Trading Company. According to the log, he had been the guardian of nine Osage, including Anna Brown and her mother, Lizzie. As I went down the list, I noticed that a third Osage Indian had died under Mathis’s guardianship, and so had a fourth, and a fifth, and a sixth. Altogether, of his nine listed wards, seven had died. And at least two of these deaths were known to be murders.

Though the case is long over and the people are dead, it still affects the community to this day. Large amounts of their family members were killed only so that others could be enriched by the land the Osage owned.