For those unfamiliar, it was the first known casualty in the UFO phenomenon. Kentucky ANG pilot Captain Mantell died while flying a North American F-51D Mustang on January 7, 1948, in pursuit of an unidentified flying object.
Tidbits for UFO researchers...
Item #1: The aircraft type designation was officially changed from 'P' (Pursuit) to 'F' (Fighter), shortly after the USAAF ('Army Air Corps') became the USAF in 1947. Therefore, any 'P' aircraft still in service became 'F' aircraft. This did not affect aircraft type designations in other branches of the military.
Item #2: I've read that Capt. Thomas Mantell, Jr., 'was new to the Mustang' or equivalent remark. I can't verify what types he flew, but he had over 2,100 hours flying experience and was a decorated WW2 air combat veteran. Captain Mantell's 4-ship flight, which originated in Georgia, was called a 'training flight', as are all flights that don't involve test, combat or ferry missions, but the term 'training flight' doesn't necessarily indicate lack of experience.
Item #3: The direct cause of death, officially, was ground impact. One eyewitness claimed the plane 'blew up'; maybe it did, but no evidence of the aircraft being attacked exists, the airplane was fueled by highly volatile Aviation-grade gasoline, and if its airspeed exceeded its rated limit, it could have literally flown apart. Another eyewitness testified he saw the plane spiral to the ground but didn't elaborate on its apparent structural integrity.
Item #4: ALL sightings of the UFO in question were visual. After corroboration by the Kentucky State Police, Godman Field was notified, and 3 of 4 planes in Mantell's flight began pursuit.
Item #5: To satisfy those who would say 'you don't want to believe!', well, I do believe. I believe that USAF Captain Tom Mantell died in the line of duty, pursuing an object he and/or his wingmen could not identify, regardless of what that object actually was. I believe the major contributing factors were foolhardiness (typical of 'fighter jocks', i.e. he knew he was low on oxygen but elected to pursue past human capability), and probably also developed target fixation (a condition where the pilot becomes so intent on his objective that everything else is blocked out). I believe as a result of those factors, Captain Mantell developed hypoxia, blacked out, and did not recover in time to regain control of his aircraft.
Sources: Wikipedia, The Kentucky National Guard Museum, daviddarling.info/encyclopedia, The Black Vault Encyclopedia Project, aths.com/rootsweb/mantel.txt