What were Project Sign and Project Grudge?

“Project Sign (1948) was the U.S. Air Force's first formal UFO investigation; it produced the 'Estimate of the Situation' concluding extraterrestrial origin was likely. The estimate was rejected and the program reorganized as Project Grudge (1949–1952), then Project Blue Book.”

Project Sign was the U.S. Air Force's first formal UFO investigation, established at Wright-Patterson Air Force Base in late 1947 and active through 1948. It produced an internal classified document called the "Estimate of the Situation," which reportedly concluded that the most plausible explanation for the recurring sightings was extraterrestrial origin. Air Force Chief of Staff General Hoyt S. Vandenberg rejected the estimate as unsupported by evidence and ordered the document destroyed; no copy is in the public record. The investigation was renamed Project Grudge in 1949 with a more skeptical institutional posture; Grudge was largely a debunking exercise and was wound down in 1952. The program was then reorganized as Project Blue Book (1952–1969), which catalogued 12,618 cases. Project Sign and Project Grudge predate PURSUE Release 01 by decades, but the December 1947 Wright Field AMC memorandum (PURSUE-023) is the immediate predecessor document.