Tham khảo Estimate of the Situation

Michael D. Swords, Project Sign and the Estimate of the Situation in Journal of UFO Studies, 2000 Đọc trực tuyến
Jerome Clark, The UFO Book: Encyclopedia of the Extraterrestrial, Visible Ink, 1998 ISBN 1-57859-029-9
David Michael Jacobs, The UFO Controversy In America, Indiana University Press, 1975 ISBN 0-253-19006-1
Diana Palmer Hoyt, Ufocritique: UFO's, Social Intelligence and the Condon Committee, Master's Thesis, Virginia Polytechnic Institute, 2000 Đọc trực tuyến
J. Allen Hynek, The UFO Experience: A Scientific Inquiry, McGraw-Hill Contemporary, 1972 ISBN 978-0809291304
Kevin D. Randle, UFO Casebook, Warner Books, 1989 ISBN 0-446-35715-4
Jenny Randles, Peter Houghe, The Complete Book of UFOs: An Investigation into Alien Contact and Encounters, Sterling Publishing Co, 1994, ISBN 0-8069-8132-6
Edward J. Ruppelt, The Report on Unidentified Flying Objects, 1956, Doubleday & Company Đọc trực tuyến Chương 2 Lưu trữ 2005-09-20 tại Wayback Machine
Michael D. Swords, UFOs, the Military, and the Early Cold War in UFOs and Abductions: Challenging the Borders of Knowledge Bản mẫu:Pp., David M. Jacobs editeur, University Press of Kansas, 2000

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