Tuesday’s blast happened after a warehouse storing ammonium nitrate caught fire, exploding with such force it was felt 150 miles (240km) away on the island of Cyprus.
The substance had reportedly been seized from a ship seven years ago and appears to have been held at the port ever since.
“It was caused by 2,750 tonnes of ammonium nitrate stored unsafely in a warehouse” Lebanon president said.
At least 100 people are known to have died in the blast, which injured thousands more.
The port appears to have been entirely destroyed, while buildings across the city sustained damage.