Jason Isbell and Sturgill Simpson have signed on for roles in Martin Scorsese's upcoming Killers of the Flower Moon, a film adaptation of a book of the same name by American journalist David Grann.

Deadline reports the news of Isbell and Simpson's additions to the movie, which has also added William Belleau (The Twilight Saga: EclipseLoudermilk) and Louis Cancelmi (Boardwalk EmpireBlue Bloods) to its cast. Isbell will portray Bill Smith, a nemesis to Ernest Burkhart (a previously cast Leonardo DiCaprio), while Simpson will play Henry Grammer, a rodeo champion and bootlegger.

Isbell's role in Killers of the Flower Moon is his first major acting role, though he did play a small part in the Deadwood movie. Simpson, meanwhile, has previously appeared in The Hunt, Queen & Slim and The Dead Don't Die, to which he also contributed a song.

The book Killers of the Flower Moon, released in 2017, digs into the serial murders of several wealthy Osage Nation members, to whom a court awarded the rights to profits made from oil deposits on their land in Osage County, Okla. Known as the Reign of Terror, the early-1920s murders number at least 20, though Grann writes that there may have been hundreds more.

Cattle farmer William Hale -- a prominent figure on the Osage Indian Reservation, where he built his ranch -- and his nephews, Ernest and Bryan Burkhart, conspired to commit the murders, a court ruled in the late '20s. Hale was sentenced to life in prison after being convicted of only a single one of the murders -- that of Henry Roan, the cousin of Anna Brown, herself the sister of Mollie Kyle, a native Osage and the wife of Ernest Burkhart -- but was released on parole in 1947.

Ernest Burkhart, too, was sentenced to life in prison, but released in 1959 and pardoned in 1966. Bryan Burkhart, meanwhile, became a witness for the state and avoided serving any time.

In the Scorsese-directed film, Robert De Niro is set to play Hale, while Belleau will play Roan. Cara Jade Myers has been cast as Anna Brown, and Lily Gladstone is playing Mollie Kyle.

Killers of the Flower Moon does not yet have a premiere date. It is an Apple Original Film.

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