Three workers were killed in an explosion and fire that occurred on May 3 at the Waukegan, Illinois, plant of AB Silicones. The site is north of Chicago.

On May 7, recovery crews were still searching the rubble from the destroyed plant for the body of a fourth worker also believed dead. Three other employees were hospitalized, according to local media. Their conditions were unknown.

The company manufactures specialty silicones for application in products such as adhesives, sealants and coatings. Police said the blast is being regarded as an accident. The cause has not yet been determined, however.

In a statement expressing sympathy for the victims, their families and company employees, the plant’s general manager, Mac Penman, thanked firefighters and said management would work with them to secure the scene and help them complete their investigation.