In addition to the dead, Waukegan Police Commander Joe Florip said four workers were transported to the hospital from the scene with "moderate injuries" after the explosion.
Authorities determined that nine people were inside the building at the time of the explosion. Two had been unaccounted for after the body of one employee was discovered Saturday morning, Waukegan Fire Marshal Steven Lenzi said.
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"As the evening went on we realized there was more hazardous materials involved, we realized we were going to have to be searching in a building with 90 percent damage to it," Lenzi said.
Fire, police, and paramedic personnel are working diligently at this scene. Again, please stay out of the area and let the first-responders work. Additional information regarding the explosion and fire will be released by Waukegan officials as the details become available. pic.twitter.com/RtA7MjTVxt
Police have confirmed that the explosion took place at AB Specialty Silicones, a 30,000-square-footmanufacturing and distribution company specializing in silicone-based ingredients.
Hours after the initial blast, crews continued to fight the blaze and urged residents to stay clear of the scene.
Lenzi said at least five other buildings in the area have also been damaged. The blast likely caused nearly $1 million in damage, he said.
Hazardous materials technicians became involved as scene conditions worsened and other products from the plant caught on fire, the fire marshal said. Representatives from Illinois Environmental Protection Agency were also on the scene.
The explosion happened near the Gurnee-Waukegan border, which is about 35 miles north of Chicago.
“Something exploded,” resident Emily Laughlin told NBC Chicago. “It looked like it was a building but they stopped everyone from getting closer.”
Neighbor Zach Matusek told the station, "It kind of felt like a freight train hitting the complex. It was almost enough to buckle your knees, it was pretty rough."
Others in the area posted photos and video of the blast online and described it as a “sonic boom.”
Some as far away as southern Wisconsin reported feeling the explosion, the station reported. Power was knocked out to nearly 1,000 residents and windows were shattered across the area.
Jamie Knodel is a Los Angeles-based breaking news editor for NBC News Digital.