Lebanese President Michel Aoun said on Friday the country’s investigation into the biggest blast in Beirut’s history would examine whether it was caused by a bomb or other external interference.

“The cause has not been determined yet. There is a possibility of external interference through a rocket or bomb or other act,” Aoun was quoted by Reuters as having told local media.

Aoun, who had previously said explosive material was stored unsafely for years at the port, said the investigation would also weigh if the blast was due to negligence or an accident.

20 people had been detained so far, he added.

One source said an initial probe blamed negligence.

Lebanese authorities said Tuesday’s blast, which killed at least 137 people and injured nearly 5,000 others, was triggered by a fire igniting 2,750 tons of ammonium nitrate negligently stored in a warehouse at Beirut's port since 2013.