(Photo: Agencies)

Three people missing after an overnight explosion and fire at an Illinois silicone plant are believed dead, authorities said Saturday.

Waukegan Fire Marshal Steven Lenzi said crews suspended their search for three employees due to concerns about the stability of the structure.

Lenzi said it’s “not likely” that the three survived the Friday night explosion at AB Specialty Silicones in the Waukegan, about 50 miles (80 kilometers) north of Chicago. The coroner was on scene and crews are classifying the search as a recovery, he said.

“The conditions are really rough in there,” Lenzi said. “There’s a lot of damage. There was a lot of fire throughout.”

Nine employees were inside the plant when the explosion occurred. Four were taken to hospitals and two declined treatment. Authorities have not identified the employees.

Lenzi said the cause of the explosion has not been determined, and it could be days before crews are able to resume their search because heavy piping and other materials that were inside the 30,000-foot structure pose a danger to firefighters and investigators.

He said the plant typically operates 24 hours a day, mixing chemicals and other materials to make silicone.

The state fire marshal is investigating, along with the Occupational Safety and Health Administration.

Residents in the area said they heard the explosion and felt their homes shake. Five buildings nearby were damaged, with glass on windows and doors shattering, Lenzi said.

Nancy Carreno, who lives near the plant, told The Chicago Tribune the explosion didn’t break any windows but it was loud. She said electricity flickered then came back on.

Lenzi says more than 100 firefighters from at least 30 departments responded to the scene.