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“The cause of the explosions was not immediately determined,[5] although state media initially reported them taking place at a fireworks warehouse, while others placed them at an oil storage or chemical storage facility.[26][38][39]

“There were warehouses in the port that stored explosives and chemicals including nitrates, common components of fertilizers and explosives.[40] The Director General of Public Security, Major general Abbas Ibrahim, said the explosion was caused by the ammonium nitrate that was confiscated from the Rhosus.[41][14] The 2,750 tonnes (3,030 short tons) of ammonium nitrate was the equivalent to around 1,155 tonnes of TNT (4,830 gigajoules).[42]

“LBCI said that, according to attendees of a Higher Defence Council briefing, the fire was ignited by workers welding a door at a warehouse.[43][44][f] A former port worker told The Guardian: “There were 30–40 nylon bags of fireworks inside warehouse 12”

https://en.wikipedia.org/wiki/2020_Beirut_explosions#Cause