The known death toll from an explosion at a German industrial park grew to five on Thursday after authorities said the bodies of three more people were recovered from the chemical manufacturing site.

Two people are still missing following Tuesday’s explosion at the waste management facility of the Chempark industrial park, located in the city of Leverkusen, near Cologne.

The cause of the blast, which also injured 31 people, is not yet known. Police began their on-site investigation on Thursday, German news agency dpa reported.

Currenta, the company that operates the industrial park, said the explosion was linked to storage tanks filled with solvents.

The explosion sent a large black cloud of smoke into the air and ignited a blaze that took firefighters almost four hours to extinguish.

The environmental authority for the state of North Rhine-Westphalia, where Leverkusen is located, said on Wednesday that the smoke contained dioxin, polychlorinated biphenyl and furan compounds.

However, it could not yet say how high the concentrations of the chemicals were, dpa reported.

Currenta said pollution measurements had so far not detected any elevated levels of hazardous substances in the air.

City officials warned people not to let children play outside, use outside pools or eat fruit and vegetables from their gardens for a few days.