BERLIN (Reuters) -An explosion in a German industrial park on Tuesday killed at least one person and injured 31 others, setting off a fierce blaze that sent a pall of smoke over the western city of Leverkusen. Four people were still missing.

Emergency services took three hours to extinguish the fire at the Chempark site, home to chemicals companies Bayer and Lanxess, that flared up after the blast at 9:40 a.m. (0740 GMT), park operator Currenta said.

“We are deeply shaken by the tragic death of one colleague,” said Chempark chief Lars Friedrich, adding that a search was underway for the four missing people.

Police said five of the 31 injured people were affected seriously enough to need intensive care.

“This is a tragic moment for the city of Leverkusen,” said Uwe Richrath, mayor of the city, which lies north of Cologne.

The area and surrounding roads were sealed off for much of the day.

Police told residents living nearby to stay indoors and shut doors and windows in case there were toxic fumes. Currenta said locals should also turn off air conditioning systems while it measured the air around the site for possible toxic gases.

Chempark’s Friedrich said it was not clear what had caused the explosion, which led to a fire starting in a tank containing solvents.

“Solvents were burned during the incident, and we do not know precisely what substances were released,” Friedrich added. “We are examining this with authorities, taking samples.”

Sirens and emergency alerts on the German civil protection agency’s mobile phone app warned citizens of “extreme danger.”

Leverkusen is less than 50 km (30 miles) from a region hit last week by catastrophic floods that killed at least 180 people.

More than 30 companies operate at the Chempark site in Leverkusen, including Covestro, Bayer, Lanxess and Arlanxeo, according to its website.

Bayer and Lanxess in 2019 sold Chempark operator Currenta to Macquarie Infrastructure and Real Assets for an enterprise value of 3.5 billion euros ($4.12 billion).

($1 = 0.8492 euros)

(Reporting by Maria Sheahan, Madeline Chambers, Caroline Copley; Editing by William Maclean, Edmund Blair and Gareth Jones)