In the 1920s, the Osage were considered the richest people per capita in the world, thanks to oil wells on their land. But in the early 1920s, about two dozen tribal members were killed mysteriously. Author David Grann delves into those deaths and the conspiracy behind them in his book “Killers of the Flower Moon: The Osage Murders and the Birth of the F.B.I.” Host Geoff Norcross spoke with Grann in 2017 in front of an audience at the Portland Book Festival.