(CNN) -- An explosion rocked parts of an Illinois town Friday night, leaving four people injured and three others unaccounted for, authorities said.
The blast 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. local time, said Cmdr. Joe Florip, a Waukegan police spokesman.
Three employees are unaccounted for, Waukegan Fire Marshal Steven Lenzi said.
Florip said he would "categorize this as a massive explosion."
"Many neighboring properties are going to have damage," he said.
The fire has been extinguished, and hazardous materials technicians and other teams are on the scene and searching, Lenzi said. No cause has been determined, and the state fire marshal will assist with an investigation, he said.
Florip said the plant was open at the time of the explosion.
Lenzi said everyone affected was an employee working at the time. Four were taken to area hospitals with moderate to serious injuries, officials said. Two others were seen but requested no treatment.
Estimated damage to top $1 million
Damage is estimated at more than $1 million, Lenzi said. The blast affected at least five other buildings nearby.
"Fire, police, and paramedic personnel are working diligently at this scene," the Lake County Sheriff's Office said on Twitter. "Please stay out of the area and let the first responders work."
The office tweeted earlier it was aware of "a very loud explosion sound" and "ground shaking."
"While we have not been notified of any specific danger, it is recommended that local residents stay inside with windows closed with heat/air systems off at this time," Illinois state Rep. Joyce Mason said on Facebook.
Authorities later said they had no concern about air quality or a need to shelter in place.
Lenzi recalled one "fire instance" there over the last 10 years. The plant has been "very responsive" and was "safety cautious after the incident from the previous fire," he said. "We have had no instances as far as code violations or anything like that with the plant."