In the early twentieth century, the members of the Osage Nation became the richest people per capita in the world, after oil was discovered under their reservation, in Oklahoma. Then they began to be mysteriously murdered off. In 1923, after the death toll reached more than two dozen, the case was taken up by the Bureau of Investigation, then an obscure branch of the Justice Department, which was later renamed the Federal Bureau of Investigation. The case was among the F.B.I.’s first major homicide investigations. After J. Edgar Hoover was appointed the bureau's director, in 1924, he sent a team of undercover operatives, including a Native American agent, to the Osage reservation.

David Grann, a staff writer at the magazine, has spent nearly half a decade researching this submerged and sinister history. In his new book, “Killers of the Flower Moon: The Osage Murders and the Birth of the F.B.I.,” which is being published by Doubleday, in April, he shows that the breadth of the killings was far greater than the Bureau ever exposed. This exclusive excerpt, the book's first chapter, introduces the Osage woman and her family who became prime targets of the conspiracy.

In April, millions of tiny flowers spread over the blackjack hills and vast prairies in the Osage territory of Oklahoma. There are Johnny-jump-ups and spring beauties and little bluets. The Osage writer John Joseph Mathews observed that the galaxy of petals makes it look as if the “gods had left confetti.” In May, when coyotes howl beneath an unnervingly large moon, taller plants, such as spiderworts and black-eyed Susans, begin to creep over the tinier blooms, stealing their light and water. The necks of the smaller flowers break and their petals flutter away, and before long they are buried underground. This is why the Osage Indians refer to May as the time of the flower-killing moon.

On May 24, 1921, Mollie Burkhart, a resident of the Osage settlement town of Gray Horse, Oklahoma, began to fear that something had happened to one of her three sisters, Anna Brown. Thirty-four, and less than a year older than Mollie, Anna had disappeared three days earlier. She had often gone on “sprees,” as her family disparagingly called them: dancing and drinking with friends until dawn. But this time one night had passed, and then another, and Anna had not shown up on Mollie’s front stoop as she usually did, with her long black hair slightly frayed and her dark eyes shining like glass. When Anna came inside, she liked to slip off her shoes, and Mollie missed the comforting sound of her moving, unhurried, through the house. Instead, there was a silence as still as the plains.

Mollie had already lost her sister Minnie nearly three years earlier. Her death had come with shocking speed, and though doctors had attributed it to a “peculiar wasting illness,” Mollie harbored doubts: Minnie had been only twenty-seven and had always been in perfect health.

Like their parents, Mollie and her sisters had their names inscribed on the Osage Roll, which meant that they were among the registered members of the tribe. It also meant that they possessed a fortune. In the early eighteen-seventies, the Osage had been driven from their lands in Kansas onto a rocky, presumably worthless reservation in northeastern Oklahoma, only to discover, decades later, that this land was sitting above some of the largest oil deposits in the United States. To obtain that oil, prospectors had to pay the Osage in the form of leases and royalties. In the early twentieth century, each person on the tribal roll began receiving a quarterly check. The amount was initially for only a few dollars, but over time, as more oil was tapped, the dividends grew into the hundreds, then the thousands of dollars. And virtually every year the payments increased, like the prairie creeks that joined to form the wide, muddy Cimarron, until the tribe members had collectively accumulated millions and millions of dollars. (In 1923 alone, the tribe took in more than thirty million dollars, the equivalent today of more than four hundred million dollars.) The Osage were considered the wealthiest people per capita in the world. “Lo and behold!” the New York weekly Outlook_ _exclaimed. “The Indian, instead of starving to death . . . enjoys a steady income that turns bankers green with envy.”

The public had become transfixed by the tribe’s prosperity, which belied the images of American Indians that could be traced back to the brutal first contact with whites—the original sin from which the country was born. Reporters tantalized their readers with stories about the “plutocratic Osage” and the “red millionaires,” with their brick-and-terra-cotta mansions and chandeliers, and with their diamond rings, fur coats, and chauffeured cars. One writer marvelled at Osage girls who attended the best boarding schools and wore sumptuous French clothing, as if “une très jolie demoiselle_ _of the Paris boulevards had inadvertently strayed into this little reservation town.”

At the same time, reporters seized upon any signs of the traditional Osage way of life, which seemed to stir in the public’s mind visions of “wild” Indians. One article noted a “circle of expensive automobiles surrounding an open campfire, where the bronzed and brightly blanketed owners are cooking meat in the primitive style.” Another documented a party of Osage arriving at a ceremony for their dances in a private airplane—a scene that “outrivals the ability of the fictionist to portray.” Summing up the public’s attitude toward the Osage, the Washington Star_ _said, “That lament, ‘Lo the poor Indian,’ might appropriately be revised to, ‘Ho, the rich red-skin.’ ”

Gray Horse was one of the reservation’s older settlements. These outposts—including Fairfax, a larger, neighboring town of nearly fifteen hundred people, and Pawhuska, the Osage capital, with a population of more than six thousand—seemed like fevered visions. The streets clamored with cowboys, fortune seekers, bootleggers, soothsayers, medicine men, outlaws, U.S. marshals, New York financiers, and oil magnates. Automobiles sped along paved horse trails, the smell of fuel overwhelming the scent of the prairies. Juries of crows peered down from telephone wires. There were restaurants, advertised as cafés, as well as opera houses and polo grounds.

Although Mollie didn’t spend as lavishly as some of her neighbors did, she had built a beautiful, rambling wooden house in Gray Horse near her family’s old lodge of lashed poles, woven mats, and bark. She owned several cars and had a staff of servants—the Indians’ pot-lickers, as many settlers derided these migrant workers. The servants were often black or Mexican, and in the early nineteen-twenties a visitor to the reservation expressed contempt at the sight of “even whites” performing “all the menial tasks about the house to which no Osage will stoop.”

Mollie was one of the last people to see Anna before she vanished. That day, May 21st, Mollie had risen close to dawn, a habit ingrained from when her father used to pray every morning to the sun. She was accustomed to the chorus of meadowlarks and sandpipers and prairie chickens, now overlaid with the pock-pocking_ _of drills pounding the earth. Unlike many of her friends, who shunned Osage clothing, Mollie wrapped an Indian blanket around her shoulders. She also didn’t style her hair in a flapper bob but, instead, let her long, black hair flow over her back, revealing her striking face, with its high cheekbones and big brown eyes.

Mollie Burkhart. PHOTOGRAPH COURTESY DOUBLEDAY

Her husband, Ernest Burkhart, rose with her. A twenty-eight-year-old white man, he had the stock handsomeness of an extra in a Western picture show: short brown hair, slate-blue eyes, square chin. Only his nose disturbed the portrait; it looked as if it had taken a barroom punch or two. Growing up in Texas, the son of a poor cotton farmer, he’d been enchanted by tales of the Osage Hills—that vestige of the American frontier where cowboys and Indians were said to still roam. In 1912, at the age of nineteen, he’d packed a bag, like Huck Finn lighting out for the Territory, and went to live with his uncle, a domineering cattleman named William K. Hale, in Fairfax. “He was not the kind of a man to ask you to do something—he told you,” Ernest once said of Hale, who became his surrogate father. Though Ernest mostly ran errands for Hale, he sometimes worked as a livery driver, which is how he met Mollie, chauffeuring her around town.