BEIRUT — The head of Lebanon's customs authority was formally arrested on Monday after being questioned over the massive explosion in Beirut earlier this month, the state-run National News Agency reported.

The investigation is focused on why nearly 3,000 ton of explosive ammonium nitrate was being stored at the city's port. The ignition of the stockpile caused an explosion that tore through the capital, killing at least 180 people and wounding 6,000.

Thirty people are still missing after the Aug. 4 blast, which caused an estimated $10 billion to $15 billion in damage.

Documents that surfaced after the blast, the single most destructive in Lebanon’s history, showed that officials have known for years that 2,750 ton of ammonium nitrate were stored in a warehouse at the port and knew about the dangers.

Judge Fadi Sawwan questioned customs chief Badri Daher, who was detained days after the blast, for 4-1/2 hours in the presence of his two lawyers before issuing the arrest warrant, the agency said. Daher will remain in custody as the investigation continues.

NNA said that after questioning Daher, Sawwan headed to the scene of the blast to survey the damage and will later return to question Hassan Koraytem, who was the top port official until the day of the blast.

Lebanese President Michel Aoun said the probe into the devastating blast is “very complex” and would not be completed quickly. Aoun said the probe is divided into three parts. The first aims to determine the circumstances surrounding the cargo, the second where it came from and who shipped it, and the third to find who was responsible for handling and securing it.

Aoun said the FBI and French investigators were helping because “they, more than us, have the capability and ability to find out the details of what got the ship here, what is the source and who owns it.” — Agencies