It’s not clear what started the original explosion that seemed to involve fireworks that then appears to have set off the amonium nitrate.

Via BBC:

Lebanon’s Prime Minsiter Hassan Diab has said it is “unacceptable” that 2,750 tonnes of ammonium nitrate were being stored in a warehouse.

“I will not rest until we find the person responsible for what happened so we can hold them to account and impose the most severe punishment,” the prime minister was quoted as saying by an official Twitter account.

“It is unacceptable that a shipment of 2,750 tonnes of ammonium nitrate has been present for six years in a warehouse, without taking preventive measures and endangering the safety of citizens”.