Waukegan, Ill. — An explosion at an Illinois silicone plant Friday killed two people and left four others injured, authorities said. Two others are missing and officials said it is unlikely they survived, CBS Chicago reports.

At least one of the people killed was a worker at the plant, CBS Chicago reports.

Emergency crews in the city of Waukegan suspended the search for the missing employees due to hazardous material and the structural integrity of the building. Waukegan is located about 40 miles north of Chicago.

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"Our thoughts and prayers go out to the family and friends of all involved in this horrific incident, " Waukegan Fire Marshal Steven Lenzi said in a statement. "Our personnel worked tirelessly through the night to control this scene with help from many neighboring agencies."

It wasn't immediately clear what caused the explosion, which damaged at least five other buildings in the area. Officials estimated the damage would cost more than $1,000,000.

AB Specialty Silicones, which owns the plant, manufactures and distributes various grades of silicone products, CBS Chicago reported.

The Lake County Sheriff's Office tweeted Friday that they were aware of a "very loud explosion" and "ground shaking" felt near the Gurnee-Waukegan border. They urged people to stay away from the area to let first responders work.