Killers of the Flower Moon by David Grann

It sounds like you're describing David Grann's book "Killers of the Flower Moon: The Osage Murders and the Birth of the FBI." This non-fiction work delves into the chilling true story of the Osage murders in the 1920s, where members of the Osage Nation in Oklahoma were systematically killed for their oil wealth. The book explores the investigation led by the FBI, then a fledgling organization, and the efforts of Tom White, a former Texas Ranger, to uncover the conspiracy behind these murders.

The story is a blend of true crime and history, highlighting a dark chapter in American history and the birth of federal law enforcement's efforts to tackle such crimes. It has received critical acclaim for its meticulous research and compelling narrative style.