49. Gerald K. Haines. "CIA's Role in the Study of UFOs, 1947-90" Studies in Intelligence, Semiannual Edition, No. 1 (1997). Available online (see “Early CIA Concerns, 1947-52”) from: https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/csi-studies/studies/97unclass/ufo.html#top. Also from: http://www.fas.org/sgp/library/ciaufo.html.