Somali intelligence officials say two airport workers handled a laptop that concealed a bomb that later exploded in a passenger plane.
In a video made public on Sunday by officials, one airport worker takes the laptop and hands it to another employee.
The employees then hand it over to a man who was killed when the laptop explosion blew a hole in the plane's fuselage, said Abdisalam Aato, a spokesman for the Somali Prime Minister.
Both workers have been arrested.
Somali officials identified the lone fatality as suspect Abdullahi Abdisalam Borleh. He was sucked out of the airliner through the hole from the blast Tuesday.
Bomber knew where to sit
Investigators suspect Borleh, a Somali national, carried a laptop computer with a bomb concealed onto the plane, according to a source familiar with the investigation.
He knew precisely where to sit and how to place the device to maximize damage, the source told CNN.
Given the placement, the blast likely would have set off a catastrophic secondary explosion in the fuel tank if the aircraft had reached cruising altitude, the source said.
But the explosion happened at a lower altitude, between 12,000 feet and 14,000 feet, killing the Somali national and injuring two others.
Though preliminary tests showed the bomb contained a military grade of the explosive TNT, the source said, it failed to bring down Daallo Airlines Flight 3159. The pilot turned around and landed the Airbus safely in Mogadishu.
Somalia asked U.S. officials for help with investigations, and several FBI agents are on the ground assisting in Mogadishu, the spokesman said.
"This was a sophisticated attack ... so we reached out to our international partners," Aato said.
Militants behind attack
Investigators believe the attack was orchestrated by Al-Shabaab, although they are not certain Borleh was a direct member of the group, according to the source. No group immediately claimed responsibility.
Al-Shabaab is an al Qaeda affiliate, though some of its it has factions have declared loyalty to ISIS. It has been responsible some of the deadliest violence in recent years in Somalia and surrounding nations, including Kenya and Uganda.
At least 20 people have been arrested in connection with the blast aboard the plane, the spokesman said.