The blast killed at least 135 people and injured more than 4000 others. A two-week state of emergency has begun.
President Michel Aoun said the blast was caused by 2750 tonnes of ammonium nitrate stored unsafely in a warehouse.
Customs chief Badri Daher said his agency called for the chemical to be removed, but "this did not happen".
"We leave it to the experts to determine the reasons," he said.
Ammonium nitrate is used as a fertiliser in agriculture and as an explosive.