Officials said they have little hope of finding five missing workers alive, a day after an explosion at an industrial park for chemical companies in western Germany killed at least two people and injured 31 others.

Tuesday’s explosion at the waste management facility of the Chempark site – in the city of Leverkusen, near Cologne – sent a large black cloud of smoke into the air and ignited a blaze that took firefighters almost four hours to extinguish.

“The events of yesterday shocked us all,” Frank Hyldmar, chief executive of Currenta, which operates the Chempark industrial park, said.

“At least two people lost their lives,” he added.

“Five are currently still missing. We no longer have much hope of finding them alive.”

He added that of the 31 injured, one person remained in a life-threatening condition.

The cause of the explosion is not yet known. Police are expected to begin their investigation in the coming days.

Currenta said the blast was linked to storage tanks filled with solvents.

Due to the thick smoke, authorities told residents on Tuesday to stay indoors, and closed several major roads for hours. City officials also warned people not to let children play outside, use outside pools, or eat fruit and vegetables from their gardens for the next few days.

The environmental authority for the state of North Rhine-Westphalia, where Leverkusen is located, said on Wednesday that the explosion’s smoke contained dioxin, polychlorinated biphenyl and furan compounds. But it could not yet say how strong the concentration of these chemicals were, German news agency dpa reported.

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

Leverkusen is home to Bayer, one of Germany’s largest chemical companies and one of the biggest employers in the region. The city has about 163,000 residents, many of whom work for Bayer.

The industrial park is located close to the river Rhine.