Project Sign's final report, published in early 1949, stated that while some UFOs appeared to represent actual aircraft, there was not enough data to determine their origin. Project Sign was followed by another project, Project Grudge.
Project Sign was first disclosed to the public in 1956 via the book The Report on Unidentified Flying Objects by retired Air Force Captain Edward J. Ruppelt. The full files for Sign were declassified in 1961.
- Blum, Howard, Out There: The Government's Secret Quest for Extraterrestrials. Simon and Schuster, 1990
- Edward J. Ruppelt (1956). The Report on Unidentified Flying Objects, Doubleday & Co. online Archived 2005-09-20 at the Wayback Machine
- Hoyt, Diana Palmer (2000). UFOCritique: UFOs, Social Intelligence, and the Condon Committee, p.16 Archived 2007-03-17 at the Wayback Machine; Master's thesis, Virginia Polytechnic Institute and State University.
- Dolan, Richard M. (2002) UFOs and the National Security State: Chronology of a Cover-up 1941–1973. Hampton Roads Publishing Company, ISBN 1-57174-317-0
- Peebles, Curtis (1994). Watch the Skies! - A Chronicle of the Flying Saucer Myth. Smithsonian, ISBN 1-56098-343-4.
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