While flying his small plane in June of 1947, businessman and civilian pilot Kenneth Arnold reported seeing nine objects moving at high speeds through the skies over Washington’s Mount Rainier. Widely publicized reports of Arnold’s experience, followed by an increasing number of reported UFO sightings, led the U.S. Air Force to begin investigating the incidents with Operation Sign in 1948.

The initial investigation resulted in the formation of Project Blue Book in 1952, which became the longest-running of the U.S. government’s official inquiries into UFO sightings, compiling reports on more than 12,000 sightings or related events from 1952 until its termination. The project, headquartered at Wright-Patterson Air Force Base, Ohio, was terminated on Dec. 17, 1969. Of a total of 12,618 sightings reported to Project Blue Book, 701 remained "unidentified."

SOURCES

https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/ufo/usaf_fact_sheet_95_03.pdf

https://www.dni.gov/files/ODNI/documents/assessments/Prelimary-Assessment-UAP-20210625.pdf

https://www.archives.gov/research/military/air-force/ufos

https://www.af.mil/About-Us/Fact-Sheets/Display/Article/104590/unidentified-flying-objects-and-air-force-project-blue-book/

https://www.esd.whs.mil/Portals/54/Documents/FOID/Reading%20Room/UFOsandUAPs/proj_b1.pdf?ver=2017-05-22-113513-837

https://vault.fbi.gov/Project%20Blue%20Book%20%28UFO%29%20/Project%20Blue%20Book%20%28UFO%29%20part%201%20of%201/view

https://www.esd.whs.mil/Portals/54/Documents/FOID/Reading%20Room/UFOsandUAPs/nas_re1.pdf?ver=2017-05-22-113513-883