In December 1947, the U.S. Air Force launched Project Sign, its first formal UFO investigation, tasked with determining whether unidentified flying objects posed a threat to national security, following a wave of public sightings after Kenneth Arnold's June 1947 report.

Project Sign operated through 1948 and investigated hundreds of sightings. Its personnel concluded the extraterrestrial hypothesis was the most credible explanation, producing a document known as the 'Estimate of the Situation,' which was rejected and reportedly destroyed by Air Force Chief of Staff General Hoyt Vandenberg.