Story highlights Investigators believe Al-Shabaab is responsible, says U.S. official

The explosion rocked Daallo Airlines Flight 3159 soon after it took off from Mogadishu

Authorities think one person fell to death

(CNN) A military grade of the explosive TNT caused this week's explosion on a Somali airliner, a source familiar with the investigation told CNN, citing an initial analysis of residue recovered from the aircraft.

The blast took place Tuesday on board a commercial plane after it set off above East Africa.

Two people were injured before the pilot landed safely, a passenger said.

Somali authorities later discovered a body near Mogadishu they believe fell from the Daallo Airlines plane.

Pictures from the ground showed a hole in one side of the airliner, just above its wing and slightly smaller than one of its doors.

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