WAUKEGAN, Ill. (CBS) — An explosion lit up the sky in Waukegan five months ago, and investigators have now called the business an “unsafe work environment with deadly consequences.”

Four employees died in the May 3 fire at the Waukegan plant.

The U.S. Department of Labor has now wants $1.6 million in penalties from the company, AB Specialty Silicones. The department’s Occupational Safety and Health Administration cited the company 12 willful federal safety violations in the fire and explosion, according to a news release.

The blast shook nearby homes and businesses. There was so much debris that search crews on the ground spent five days sifting through rubble – looking for the lost.

Five people got out safely, but four did not. Byron Biehn, Jeff Cummings, Allen Stevens, and Dan Nicklas, were all killed. Stevens escaped, but died later at the hospital.

OSHA investigators determined AB Specialty Silicones failed to keep electrical equipment and installations in compliance with OSHA electrical standards. Officials also say the company used forklifts powered by liquid propane to move volatile flammable liquids, in areas where employees handled volatile flammable liquids and gases. These factors created the potential for ignition, OSHA said.

“Employers must employ hazard recognition to protect workers from harm, especially in high hazard industries,” said Loren Sweatt, Principal Deputy Assistant Secretary of Labor for Occupational Safety and Health, in the news release. “By ignoring safety and health requirements, this employer created an unsafe work environment with deadly consequences.”

An AB Specialty Silicones spokesperson said in a written statement they are currently reviewing the citations, and that “the safety of our employees has been and will remain our top priority.”

“We cooperated fully with the agency’s investigation, and we will continue to work with the agency. It is important to recognize the scope of the OSHA report goes beyond the elements that contributed to the tragic explosion. Our internal investigation, as well as the Fire Marshal’s report, confirmed the incident was in fact an accident, and there was no willful intent involved.”

Before the blast, AB Specialty Silicones had a clean slate with OSHA and no outstanding violations with the Illinois Environmental Protection Agency.

AB Specialty Silicones now has 15 business days to comply with or contest the findings.

Fire investigators believe the blast was the result of an accident, but they still have not given a conclusive cause. Early on, we learned plant workers noticed something was wrong with a stainless steel storage vessel.