CANNES, France (AP) — It was well into the process of making "Killers of the Flower Moon" that Martin Scorsese realized it wasn't a detective story.

Scorsese, actor Leonardo DiCaprio and screenwriter Eric Roth had many potential avenues in adapting David Grann's expansive nonfiction history, "Killers of the Flower Moon: The Osage Murders and the Birth of the FBI." The film that Scorsese and company premiered Saturday at the Cannes Film Festival, however, wasn't like the one they initially set out to make.

The film, which will open in theaters in October, chronicles the series of killings that...