Lebanon's president said on Friday (August 7) that the investigation into Tuesday's (August 4) blast in Beirut would examine whether it was caused by a bomb or other external interference.
The explosion killed more than 150 people and injured at least 5,000.
President Michel Aoun has previously said explosive material - thousands of tonnes of ammonium nitrate - was stored unsafely at the port in Beirut for years.
And now he's said the investigation will also assess if the blast was due to negligence or an accident.
He added that 20 people had been detained so far.
President Trump has suggested the United States hadn't ruled out an attack as the cause of the blast.
Israel, which has fought several wars with Lebanon, was quick to deny any role.
Turkey's President Erdogan said the cause was unclear.
A Lebanese security source and local media previously said the fire that caused the blast was ignited by warehouse welding work.