Project Sign to Project Grudge: The U.S. Air Force's 1948–1949 UFO Investigation

To the point

An Air Force program started in 1948 as Project SAUCER under General Nathan Farragut Twining to study UFO sightings for national security, later becoming Project Sign and then Project Grudge, which in 1949 issued a cautious finding that some sightings might be real aircraft but most had ordinary explanations, while Edward J. Ruppelt later claimed a “Top Secret Estimate of the Situation” that UFOs were real (a claim a 1966 congressional hearing said had no basis), and Grudge dismissed sightings as misidentifications, mass hysteria, hoaxes, or psychopathology, with a May 1949 investigation tracing a lead to Glen Burnie, Maryland, to the damaged remains of Jonathan Edward Caldwell’s disk-rotor aircraft that could not account for widespread saucer reports, though photos of the broken machine still appear in UFO literature.

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Project Sign - Wikipedia

Project Sign, later Project Grudge, was a U.S. Air Force study of unidentified flying objects from 1948 to 1949—established by General Nathan Farragut Twining and initially named Project SAUCER—to collect, evaluate, and disseminate sightings for national security, followed by a 1949 Air Force paper saying some UFOs appeared to be aircraft but most cases had ordinary explanations, an alleged but uncorroborated claim of a 'Top Secret Estimate of the Situation' that UFOs were real (claimed by Edward J. Ruppelt), Grudge’s stance that reports were misidentifications, mass hysteria, hoaxes, or psychopathology, and a May 1949 lead traced to Glen Burnie, Maryland regarding Jonathan Edward Caldwell's disk-rotor aircraft whose prototypes could not account for widespread saucer reports, though photographs of the broken machine persist in UFO literature.