Tham khảo Dự_án_Sign

Tham khảo Dự_án_Sign

↑ 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 Lưu trữ 2005-09-20 tại Wayback Machine
↑ Hoyt, Diana Palmer (2000). UFOCritique: UFOs, Social Intelligence, and the Condon Committee, p.16 Lưu trữ 2007-03-17 tại Wayback Machine; Master's thesis, Virginia Polytechnic Institute and State University.

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