Lebanese officials say ammonium nitrate, a substance used as fertilizer in agriculture, likely caused the massive explosion in Beirut. The explosion killed at least 100 people and injured more than 4,000 others on Tuesday. It flattened much of the port and damaged buildings across the capital city. Lebanon’s President Michel Aoun said more than 2,700 tons of ammonium nitrate had been stored without safety measures for six years at the port. He added the government was “determined to investigate and expose what happened as soon as possible.”