NAIROBI, Kenya—Investigators believe a passenger who boarded a commercial plane in a wheelchair may have been a suicide bomber responsible for an explosion that tore a hole in the side of the jet as it took off from Somalia’s capital, a Western diplomat briefed on the probe said.

Investigators’ theory is that the man was able to use the wheelchair to bypass rigorous security screening in Mogadishu and then detonated a bomb that ripped a hole in the fuselage but failed to down the plane. The force of the blast ejected the alleged bomber from the cabin and his body fell to earth near the town of Balad, about 20 miles from Mogadishu, where it was recovered by authorities.

“An individual got onto the plane in a wheelchair and is suspected of being the suicide bomber,” said the diplomat, who is in direct contact with investigators.

The revelation comes amid mounting evidence that the blast aboard Tuesday’s Daallo Airlines flight was a terror attack. The passenger was brought on board with the wheelchair then transferred to a regular seat, the person said.

Both U.S. officials and the plane’s pilot on Wednesday said they believed a bomb exploded aboard the flight, while the Western diplomat said investigators had swabbed the area of the explosion and found the residue of TNT.