In the 1920s, the richest people per capita in the world were members of the Osage Indian nation in Oklahoma. After oil was discovered beneath their land, they rode in chauffeured automobiles, built mansions, and sent their children to study in Europe. Then, one by one, the Osage began to be killed off. Many of those who dared to investigate the killings were themselves murdered. As the death toll climbed to more than twenty-four, the FBI took up...