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.\r\nThe investigation is focused on why nearly 3,000 tons 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.\r\nThirty people are still missing after the August 4 blast, which caused an estimated $10 billion to $15 billion in damage.\r\nDocuments that surfaced after the blast, the single most destructive in Lebanon\u2019s history, showed that officials have known for years that 2,750 tons of ammonium nitrate were stored in a warehouse at the port and knew about the dangers.\r\nJudge 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.\r\nNNA 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.\r\nLebanese President Michel Aoun said the probe into the devastating blast is \u201cvery complex\u201d and would not be finished 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.\r\nAoun said the FBI and French investigators were helping because \u201cthey, 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.\u201d\r\nA nine-member team of FBI investigators landed in Beirut on Sunday, according to a Lebanese aviation official, and were believed to have joined the investigation. French investigators have been active for days at the port.\r\nPopular anger has swelled over the ruling elite\u2019s corruption and mismanagement. Lebanon\u2019s government, which is supported by the militant Hezbollah group and its allies, resigned on August 10 and continues to serve in a caretaker capacity. There are no formal consultations underway on who will replace Hassan Diab as prime minister and no likely candidate has emerged.