Japan's Coast Guard said it found what appeared to be wreckage from the tilt-rotor V-22 Osprey and one person who was later confirmed to have died some 3 km off the coast of Yakushima island.
The crash happened just before 3 p.m. (0600 GMT) with witnesses saying the aircraft's left engine appeared to be on fire as it approached an airport for an emergency landing, despite clear weather and light wind, local media reported.
The coast guard corrected the number of people on board the Osprey aircraft to six from an initially announced eight.