Air National Guard pilot Thomas Mantell, flying in pursuit of an alleged UFO, was killed when his P-51 Mustang crashed near Fort Knox, Kentucky.

The Kentucky Air National Guard (KY ANG) is the aerial militia of the U.S. state of Kentucky. It is a reserve of the United States Air Force and along with the Kentucky Army National Guard, an element of the Kentucky National Guard of the much larger United States National Guard Bureau.

On 7 January 1948, 25-year-old Captain Thomas F. Mantell, a Kentucky Air National Guard pilot, died when the P-51 Mustang fighter plane he was piloting crashed near Franklin, Kentucky, United States, after being sent in pursuit of an unidentified flying object (UFO). Pursuing the object in a steep climb and disregarding suggestions to level his altitude, Mantell lost consciousness at high altitude from lack of oxygen; his plane went into a downward spiral and crashed. The incident was among the most publicized early UFO reports. Later investigation by Edward J. Ruppelt, Head of United States Air Force's Project Blue Book found that observations from an airforce base control tower, particularly of "a parachute" and "an ice cream cone tipped with red", as well as an astronomer at Vanderbilt University who reported "a pear-shaped balloon with cables and a basket attached", suggested that Mantell died chasing a Skyhook balloon, which in 1948 was a top-secret project that he would not have known about. The Skyhook was likely launched from Clinton County AFB.

An unidentified flying object (UFO) is an object or phenomenon seen in the sky but not yet identified or explained. The term was coined when United States Air Force (USAF) investigations into flying saucers found too broad a range of shapes reported to consider them all saucers or discs. UFOs are also known as unidentified aerial phenomena or unidentified anomalous phenomena (UAP). Upon investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.