Waukegan Fire Marshal Steve Lenzi said several workers are unaccounted for, Lake and McHenry County Scanner reports.
The explosion damaged at least five other buildings in the area and damage is estimated to be in excess of $1 million, Lenzi said.
However by 11:30am, officials confirmed that the bodies of two other employees who were previously reported missing had been recovered.
Meanwhile, the search for the other two missing workers was suspended due to intense structural damage.
Officials say the cause of the explosion is still not yet known.
"Our thoughts and prayers go out to the family and friends of all involved in this horrific incident", Lenzi said in the statement.
Lenzi reported that the first arriving crews "were quickly overwhelmed" by flames at the plant, which is on the city's far northwest side near its border with Gurnee.
The agency urged people to "please stay out of the area and let the first-responders work".
The scene of an explosion at a manufacturing plant in Illinois, May 3, 2019.
Authorities recover a body at the site of a factory explosion near Chicago.
Megan Hener, who lives on Atlantic Avenue six blocks from the scene, said she and her husband, Robert, drove down to the corner of Northwestern and Sunset avenues and posted a photo on social media that shows the building gone.
The plant is directly across from an emissions testing facility, a witness told the newspaper.
The "catastrophic explosion" hit around 9:30 p.m. local time, said Cmdr. "It shook our house and shook my insides".
Employees were working inside the building at the time of the blast.
"It was leveled. It's right across from the emission testing station", she said, in reference to a state of IL facility. "Our cable TV flickered and the electricity, and then it came back on", she said.