Killers of the Flower Moon: The Osage Murders and the Birth of the FBI, by David Grann. In the 19th century the Osage nation was moved from Kansas to what was is now Oklahoma, and in the early 20th century oil was discovered on that land. Their leader at the time was very savvy, and before the presence of the oil became widely known some of the Osage had become aware that the oil was there. They sent a lawyer to Congress to negotiate ownership of the land, and part of the contract was that the Osage would own anything in the ground. Congress at the time thought it was a nothing clause and agreed to it I think almost just to humour them. Joke's on them though because once oil was discovered the Osage became the world's wealthiest people, although maybe it was the Osage on whom the joke was, because racist-ass Congress decided that the Osage weren't allowed to manage their own money, and appointed guardians to manage it. If someone wanted to spend their money they had to ask their guardian, and they were only allowed to withdraw a certain amount of it at a time. There were politicians who actually spent their time poring over bank statements to learn what the Osage members were spending their money on. Obviously, there were white men who were stupid with money too, but they were allowed to make those mistakes.
Individual Osage members were being killed; there were something like 24 over a four year period, and also killed were white people who tried to advocate on their behalf. One man went to Washington to speak with politicians about investigating the murders and was murdered there, and a lawman / detective dispatched to investigate was also murdered (on a train). Eventually J. Edgar Hoover got the nascent FBI involved, although he was trying to hire people who were educated and au fait with modern investigative methods. He still had a handful of older, Wild West-style lawmen on the books, and so he sent one in to investigate, giving him the leeway to assemble a team. This guy became the public face of the investigation despite the safety risk, and he brought in an undercover team. One of them came into town as an insurance salesman and set up a shop selling real insurance policies while investigating. I haven't got much further than this, but it seems like a widespread conspiracy between very powerful people to bump off the richest of the Osage. The book is easy to read and very informative and engaging, I highly recommend it.
