The Mantell UFO incident

Captain Thomas Mantell, a Kentucky Air National Guard pilot, died on 7 January 1948 when his fighter crashed while he was chasing an unidentified object over Kentucky.

Fort Knox News, published by the U.S. Army, says Mantell was flight leader of four fighters from a Kentucky Air National Guard squadron. They were flying north from Georgia to Louisville after a navigation exercise when Col. Guy Hix, the Godman commander, asked them to look into sightings. The Kentucky National Guard's eMuseum says the Godman Field tower directed Mantell to pursue the object.

Military.com reports that Mantell reached 15,000 feet and described the object as metallic and very large, travelling at half the speed of his plane. He kept climbing, to 22,000 feet. His wingmen dropped back because they had no oxygen equipment, and he had none either. Wikipedia says he lost consciousness from lack of oxygen, and his plane spiraled down and crashed. Mantell died in the crash near Franklin, in Simpson County, Kentucky, the eMuseum records.

Edward J. Ruppelt, in his 1956 book The Report on Unidentified Flying Objects, concluded that Mantell had been chasing a huge skyhook balloon launched from southwestern Ohio, a type Mantell had never heard of. Military.com reaches the same conclusion, calling it a secret balloon. The 2023 Fort Knox News article says questions remain. Wikipedia calls the case one of the most publicized early UFO reports.