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This document comprises official U.S. Air Force records from Project "Sign," the Air Force's early systematic investigation of unidentified flying objects. The bulk consists of Dr. Hynek's categorical evaluations from Project Grudge Report, classifying 244 incidents by astronomical probability, balloons/aircraft, rockets/flares, and unexplained sightings.

The primary detailed case is Incident #159 (Worthington, Ohio, 30 July 1948): a civilian observer reported a cylindrical object with no wings or tail surfaces, steady silver glow, no marker lights or engine noise, at approximately 3,000 feet altitude moving slowly southwest, visible for fifteen minutes. Intelligence Department correspondence requests confirmation from Douglas Leigh Sky Advertising Corporation regarding lighter-than-air operations near Columbus, Ohio during late July–early August 1948, attempting to correlate balloon operations with sightings.