UFO. United States

Mantell was an experienced pilot; his flight history consisted of 2,167 hours in the air, and he had been honored for his part in the Battle of Normandy during World War II.

On 7 January 1948, Godman Army Airfield at Fort Knox, Kentucky, received a report from the Kentucky Highway Patrol of an unusual aerial object near Madisonville, Kentucky. Reports of a westbound circular object, 250–300 feet (80–90 m) in diameter, were received from Owensboro and Irvington.

At about 1:45 p.m., Sergeant Quinton Blackwell saw an object from his position in the control tower at Fort Knox. Two other witnesses in the tower also reported a white object in the distance. Colonel Guy Hix, the base commander, reported an object he described as "very white," and "about one fourth the size of the full moon ... Through binoculars it appeared to have a red border at the bottom ... It remained stationary, seemingly, for one and a half hours." Observers at Clinton County Army Air Field in Ohio described the object "as having the appearance of a flaming red cone trailing a gaseous green mist" and observed the object for around 35 minutes. Another observer at Lockbourne Army Air Field in Ohio noted, "Just before leaving it came to very near the ground, staying down for about ten seconds, then climbed at a very fast rate back to its original altitude, 10,000 feet, leveling off and disappearing into the overcast heading 120 degrees. Its speed was greater than 500 mph (800 km/h) in level flight."

Four F-51D Mustangs of C Flight, 165th Fighter Squadron Kentucky Air National Guard—one piloted by Mantell—were already in the air and told to approach the object. Blackwell was in radio communication with the pilots throughout the event.

One pilot's Mustang was low on fuel, and he quickly returned to base. Air Force Captain Edward J. Ruppelt (the first head of Project Blue Book) notes that there was some disagreement amongst the air traffic controllers as to Mantell's words as he communicated with the tower: some sources reported that Mantell had described an object "[which] looks metallic and of tremendous size," but according to Ruppelt, others disputed whether or not Mantell actually said this.

The other two pilots accompanied Mantell in steep pursuit of the object. They later reported they saw an object, but described it as so small and indistinct that they could not identify it. Mantell ignored suggestions that the pilots should level their altitude and try to more clearly see the object.

Only one of Mantell's wingmen, Lt. Albert Clements, had an oxygen mask, and his oxygen was in low supply. Clements and the third pilot, Lt. Hammond, called off their pursuit at 22,500 feet (6,900 m). Mantell continued to climb, however. According to the Air Force, once Mantell passed 25,000 feet (7,600 m) he blacked out from the lack of oxygen (hypoxia), and his plane began spiraling back towards the ground. A witness later reported Mantell's Mustang in a circling descent. His plane crashed on a farm south of Franklin, Kentucky, on the Kentucky–Tennessee state line.

Firemen later pulled Mantell's body from the Mustang's wreckage. His seat belt was shredded, and his wristwatch had stopped at 3:18 p.m., the time of his crash. Meanwhile, by 3:50 p.m. the UFO was no longer visible to observers at Godman Army Airfield. The Mantell incident was reported by newspapers around the nation, and received significant news media attention. A number of sensational rumors were also circulated about Mantell's crash. According to UFO historian Curtis Peebles, among the rumors were claims that "the flying saucer was a Soviet missile; it was [an alien] spacecraft that shot down [Mantell's fighter] when it got too close; Captain Mantell's body was found riddled with bullets; the body was missing; the plane had completely disintegrated in the air; [and] the wreckage was radioactive." However, no evidence has ever surfaced to substantiate any of these claims, and Air Force investigation specifically refuted some claims, such as the supposedly radioactive wreckage. Captain Ruppelt wrote that "I had always heard a lot of wild speculation about the condition of Mantell's crashed F-51, so I wired for a copy of the accident report. [It] said that...Mantell's body had not burned, not disintegrated, and was not full of holes; the wreck was not radioactive, nor was it magnetized." Mantell was the first member of the Kentucky Air National Guard to die in fligh According to John Trowbridge, historian of the Kentucky National Guard, "There is a real X-Files twist to this, too. Mantell lived almost his entire life in Louisville, but he was born in a hospital in Franklin, only a few miles from where he was killed."

In 1956, Ruppelt wrote that the Mantell crash was one of three "classic" UFO cases in 1948 that would help to define the UFO phenomenon in the public mind, and would help convince some Air Force intelligence specialists that UFOs were a "real", physical phenomenon. The other two "classic" sightings in 1948 were the Chiles-Whitted UFO encounter and the Gorman dogfight.