This link provides access to FBI files on the Osage Indian Murders. There are over 3,500 pages of documents in 65 separate pdf files, dated from 1923 to the late 1930s. The documents include witness statements, agent reports on the case, memos suggesting litigation strategy, trial transcript excerpts, letters from J. Edgar Hoover to the field office, and news clippings. The documents vary in legibility and some are redacted.
This site provides an account of the case, a case timeline, transcripts of confessions, high quality images, and a well-done 8-minute video on the Reign of Terror on the Osage Reservation.
Wikipedia's account of the Reign of Terror, with numerous links and footnotes
This site provides a brief account, with several high-quality images, of the Osage Reign of Terror.
This site provides an account of the Reign of Terror that appeared on CBS Sunday Morning in 2017. The segment includes an interview with David Grann, author of Killers of the Flower Moon.
Grann, David. Killers of the Flower Moon: The Osage Murders and the Birth of the FBI (Doubleday, 2017)
A compelling account by Grann, a staff writer on the New Yorker.
"Disturbing and riveting...Grann has proved himself a master of spinning delicious, many-layered mysteries that also happen to be true...It will sear your soul." —Dave Eggers, New York Times Book Review
Grove, Fred, Flame of the Osage (Gunsmoke Westerns, 1998)
Hogan, Laurence J. The Osage Indian Murders: The True Story of a 21-Murder Plot to Inherit the Headrights of Wealthy Osage Tribe Members (Amlex, 1998)
McAuliffe, Dennis Jr. Bloodland: A Family Story of Oil, Greed, and Murder on the Osage Reservation (Council Oak Books, 1999)
Osage Tribal Murders (Amazon Video (1 hour, 22 minutes; 2017)