On January 7, 1948, the Kentucky Highway Patrol called Godman Army Airfield at Fort Knox about a large, round object in the sky. Tower staff saw it too and could not identify it. Four F-51 Mustangs of the Kentucky Air National Guard, one flown by 25-year-old Captain Thomas Mantell, were asked to take a look.
Mantell climbed after the object past 22,000 feet even though his plane had no oxygen equipment and his wingmen turned back. He lost consciousness; his plane spiralled down and crashed near Franklin, Kentucky. He was the first pilot whose death was linked to a UFO, and the case made headlines across the country.
Project Blue Book chief Edward Ruppelt later concluded that Mantell had most likely been chasing a Navy Skyhook balloon, a huge high-altitude research balloon that was secret at the time. Descriptions from the tower and from an astronomer who saw a balloon with a basket fit that explanation.