BERLIN (AP) — Rescue teams in western Germany continued searching for missing workers Wednesday at an industrial park for chemical companies where an explosion killed at least two people and injured 31 others, German news agency dpa reported.

The Tuesday explosion at the waste management facility of the Chempark site produced a fire that it took firefighters almost four hours to extinguish. It was not clear how many people were still missing.

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

The industrial park is located in the city of Leverkusen, near Cologne. After the explosion sent a large black cloud into the air, residents were asked to stay indoors and several major highways were shut down for hours.

City officials also warned people not to let children play outside, use outside pools or eat fruit and vegetables from their backyards in the coming days.

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, operated by Currenta, is located close to the banks of the Rhine river.