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This PDF collects Department of War memoranda and correspondence on Project Sign, the 1948 Air Force program investigating reports of high-performance airborne objects. The Air Force described some reports as consistent with "real" objects, cited high speed and maneuverability beyond known U.S. military aircraft, and said there was an "imperative need" for evidence to judge any national security threat.

Air Materiel Command found that observers were generally not mistaken that they had seen something airborne, but that unusual descriptions likely reflected the "limits of their personal experience." It identified "a great many" rounded objects as weather or upper air scientific balloons, and several other cases as likely celestial phenomena such as Venus. A small number lacked a "reasonable everyday explanation," but AMC found no physical evidence, no support for "vehicles from another planet," and "no conclusive proof" that unidentified flying objects other than known balloons were real aircraft.

Declassified by the Department of War under the PURSUE UAP records release (tranche of 8/7/26).
Agency: Department of War
Incident: 1947-1948
Document ID: DOW-UAP-D100