CHICAGO (CBS) — The search continues for one man still missing after a deadly explosion at the AB Specialty Silicones plant in Waukegan this weekend.

The massive blast late Friday night rocked nearby homes, sending debris flying for blocks. Three men have been confirmed dead, and another is missing and presumed dead.

Four people were taken to the hospital Friday night, including 29-year-old Allen Stevens, who died Saturday afternoon, after more than 12 hours in the burn unit at Loyola University Medical Center.

Two bodies were recovered from the rubble on Saturday and Sunday, but one more employee who was in the chemical plant at the time of the explosion remains missing.

Crews returned to the scene at 9 a.m. Monday to continue searching for that last victim.

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

“Our firefighters are working in some horrible conditions in the area. The entire building has collapsed in the area we’re searching. We’re systematically working piece by piece. That is the plan today: continue to remove heavy debris, which would be under the structure. We’re still looking for that third unaccounted for person,” he said.

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.

Lenzi said management staff inside the building Friday night were aware something wasn’t right inside one of the stainless steel processing vessels, and that’s when they began evacuating the nine people inside the plant. It’s unclear how long after they noticed the problem that this happened.

However, Lenzi said the investigation into the cause could take up to a month.

On Sunday, the Waukegan Fire Department’s technical team worked more than two hours to recover the body of one worker found in a heavily damaged section of the building. Another body was removed from the explosion site on Saturday.

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.

The explosion damaged at least five other buildings in the area. Officials estimated the total damage to be more than $1 million. At least two nearby businesses were closed on Monday as they work on repairs.

Officials said they do not yet know what caused the blast, but they believe it was accidental.

Lenzi said workers mix a lot of chemicals in the plant, and something apparently went wrong Friday night. Workers noticed a problem and began to evacuate the building before the explosion.

Autopsies were scheduled for Monday for the two bodies recovered this weekend. Authorities plan to use dental records to help identify the men.

Stevens’ friends set up an online fundraiser to start a memorial fund in his honor. They’ve raised more than $19,000 so far.

Three other employees were still in the hospital, but their conditions were not available.

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.

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.”