CHICAGO (CBS) — Lake County authorities have identified two more victims of last weekend’s deadly explosion at a chemical plant in Waukegan.

Autopsies on 53-year-old Byron Biehn, of Brighton Township, Wisconsin, and 57-year-old Jeff Cummings, of Kenosha, were inconclusive, pending the results of toxicology tests, according to the Lake County Coroner’s office.

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“We have been in close contact with both families and our deepest condolences go out to them during this difficult time. This is truly an awful tragedy of epic proportion. We continue to work on scene, in efforts to recover the final victim. Our condolences also go out to the entire AB Specialty Silicones family. We know this has been a trying time for you all, as well,” Lake County Coroner Dr. Howard Cooper said.

Cummings’ and Biehn’s bodies were recovered from the rubble of the AB Specialty Silicones factory over the weekend, following a massive blast that leveled the plant Friday night.

A third victim, 29-year-old Allen Stevens, died at the hospital Saturday afternoon, after approximately 12 hours in the burn unit at Loyola University Medical Center.

A fourth victim, whose name has not been released, is still missing, and firefighters were continuing to search the remains of the collapsed factory on Tuesday.

Three other employees were injured in the explosion.

Waukegan Fire Marshal Steve Lenzi said the search for the still-missing employee has been difficult, because the building was leveled in the blast, and what’s left at the site isn’t structurally sound.

Lenzi said 75% of the area in question has been searched. The main area is the lower portion of the building that was an office and production area, which is where investigators believe the problem started.

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Factory managers inside the building Friday night were aware something wasn’t right inside one of the stainless steel processing vessels where workers mix a lot of chemicals in the plant, and that’s when they began evacuating the nine people inside the plant, according to Lenzi. It’s unclear how long after they noticed the problem the factory exploded

Coworkers shed light on how heroic some of their lost colleagues acted.

The second shift supervisor alerted staff to evacuate after realizing something wasn’t right and refused to leave until he was certain everyone else was safe. The third shift supervisor tried saving a new employee but got trapped inside himself.

Lenzi said the investigation into the cause of the blast could take up to a month.

The explosion damaged at least five other buildings in the area.

Stevens’ friends set up an online fundraiser to start a memorial fund in his honor. They’ve raised more than $25,000 so far.

AB Specialty Silicones is a U.S. manufacturer and worldwide distributor of specialty silicone chemicals. The company has a clean slate with OSHA and no outstanding violations within the Illinois Environmental Protection Agency.

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The Silicones Environmental Health and Safety Council of North America’s handling guide says “compounds can rapidly evolve hydrogen gas and form flammable and explosive mixtures in the air.”