An explosion at an industrial park for chemical companies in Germany killed at least two people on Tuesday, with 31 others injured and several still missing hours later. Fire officials who tested the air said there did not appear to be a danger to nearby residents after authorities initially urged people to shelter inside.

The explosion at the waste management facility of the Chempark site in Leverkusen, near Cologne, sent a large black cloud into the air. It took firefighters almost four hours to extinguish the fire that took hold after the explosion.

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Germany’s Federal Office for Civil Protection and Disaster Assistance initially classified the incident as “an extreme threat.” Later on Tuesday, however, the Cologne fire department tweeted that pollution measurements “do not show any kind of abnormality.” They said the smoke had diminished but that they would continue to measure the air for toxins.

The city of Leverkusen said the explosion occurred in storage tanks for solvents.

Later Tuesday, Chempark operator Currenta said that a second fatality had been confirmed. It put the number of injured at 31 and said five employees were missing.

“Unfortunately hope of finding them alive is fading rapidly,” the head of Chempark, Lars Friedrich, said in a statement.

City officials asked all residents to stay inside until the late afternoon and warned people from outside Leverkusen to avoid the region. City officials later also warned people not to let children play outside, use outside pools or eat fruit and vegetables from their backyards in the coming days. They said experts would only be able to tell in a few days how toxic the soot from the explosion would be.

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Currenta said the explosion happened at 9:40 a.m. and then developed into a fire. It said three big tanks were affected by the explosion, but that it was too early to know the cause.

“Sirens were operated to warn residents and warning alerts were sent,” Currenta said.

Police shut down several nearby major highways for several hours.

Leverkusen is home to Bayer, one of Germany’s biggest chemical companies. It has about 163,000 residents and borders Cologne, which is Germany’s fourth biggest city and has around 1 million inhabitants. Many residents work at Bayer, which is one of the biggest employers in the region.

The chemical park is located close to the banks of the Rhine river.

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Currenta has three facilities in the region. More than 70 different companies are based at the locations in Leverkusen, Dormagen and Krefeld-Uerdingen.

The mayor of Leverkusen, Uwe Richrath, called the blast “a tragic moment for Leverkusen.”