BERLIN (AP) — Rescue teams in western Germany searched for five missing workers today at an industrial park for chemical companies a day after an explosion killed at least two people and injured 31 others.

The Tuesday explosion at the waste management facility of the Chempark site sent a large black cloud of smoke into the air and ignited a fire that took firefighters almost four hours to extinguish. The industrial park is located in the city of Leverkusen, near Cologne.

The CEO of Currenta, which operates the Chempark industrial park, said there was little hope for the five missing workers — four Currenta employees and one outside contractor.

“We have to assume that the missing people can no longer be found alive,” Frank Hyldmar told reporters in Leverkusen. He added that of the 31 injured, one person was still in a life-threatening condition.

Police said today that investigations into the cause of the explosion would begin Thursday, the German news agency dpa reported. So far, it appeared that the blast was linked to storage tanks filled with solvents.

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

Company officials said this morning that experts were measuring the air for possible toxins and analyzing soot particles from the explosion’s smoke cloud.

Leverkusen is home to Bayer, one of Germany’s biggest 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 Rhine River.