These are a few documents that reflect the attitudes of the US and other governments or defense officials regarding the UFO phenomenon.
Top-Secret 1949 Document, by Jan Aldrich
Report by the USAF Director of Intelligence to the Joint Intelligence Committee on Unidentified Aerial Objects, April 27, 1949
Air Force Advisory Panel Briefing by the Condon Committee
January 12, 1967
Observations of Anomalous Atmospheric Phenomena in the USSR: Statistical Analysis, by Lev Gindilis, D. A. Menkov, and I. G. Petrovskaya
USSR Academy of Sciences, Institute of Space Research, Report (1979)
The CIA and the U-2 Program, 1954–1974, by Gregory W. Pedlow and Donald E. Welzenbach
CIA Center for the Study of Intelligence (1998)
The French COMETA Report on UFOs and Defense, by Gildas Bourdais
A summary (July 1999) – COMETA was an unofficial report published by the French COMETA group,