(CNN) A massive blast that rocked parts of an Illinois town Friday night has resulted in three deaths, with one person still unaccounted for as of Sunday evening.

The explosion happened at AB Specialty Silicones in Waukegan, about 40 miles north of Chicago, police said. The company describes itself as a manufacturer of specialty silicone chemicals.

The "catastrophic explosion" hit around 9:30 p.m. Friday, said Cmdr. Joe Florip, a Waukegan police spokesman. The plant was open at the time of the explosion, he said, and nine people were in the building.

Waukegan Fire Marshal Steven Lenzi said everyone affected was an employee working at the time. Four were taken to area hospitals with moderate to serious injuries, and one later died from injuries, officials said. Two others were seen but requested no treatment.

Three people were unaccounted for.

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