UPDATE This article was revised on May 9, 2019, to add information about a fourth death that resulted from the AB Specialty Silicones accident. Credit: John Starks/Daily Herald via AP

Investigators are now sifting through the rubble and trying to make sense of the unusual explosion and fire on the night of May 3 that left four people dead at a US silicone manufacturing plant. The accident, at the AB Specialty Silicones plant in Waukegan, Illinois, about 60 km north of Chicago, shook nearby homes, made lights flicker, and scattered debris almost 2 km away.

According to local news accounts, the blast occurred at nightfall. Residents felt the ground shake and heard a loud boom. As local fire crews arrived, flames engulfed the plant.

First responders initially found one employee dead from the explosion. They took four others to the hospital, where one worker later died. Fire officials recovered the bodies of two missing workers by May 8, bringing the death toll from the blast to four. Nine people in all were in the plant at the time of the explosion.

In a statement posted on the AB website, General Manager Mac Penman said, “We are shocked and heartbroken by the tragedy that occurred in our plant on May 3rd. We are trying our best to support all of the members of our AB family as we attempt to process this terrible loss together.” The company plans to reopen business operations on May 13.

Waukegan authorities indicate that their inquiry into exactly what happened at AB will take some time. Joining them will be inspectors from the US Chemical Safety and Hazard Investigation Board (CSB), which said it dispatched its team to the AB site on May 5.

Explosions at silicone manufacturing facilities are not frequent, but they have happened. In 2001, specialty silicone maker Gelest lost its Tullytown, Pennsylvania, plant in a devastating explosion and fire. No loss of life occurred in that incident, but two people suffered chemical burns. The CSB did not investigate the incident, but media reports indicated that a static electrical discharge in the plant’s laboratory might have touched off the blaze.

In 2009, when C&EN visited the rebuilt plant in Morrisville, Pennsylvania, the company proudly showed off design features meant to minimize potential fire and explosion hazards.

AB manufactures a variety of functional vinyl, hydride, phenyl, and fluoro silicones used in a number of items, including personal care, dental, medical, and electronics products. But although silicon, the raw material for silicones, is not flammable, many of the organosilicons the firm makes are flammable, toxic, or corrosive. However, investigators have not said these materials are a possible cause of the AB accident at this time.

Loss of life at smaller US chemical firms is also unusual. In 1999, an explosion during the manufacture of the electronic chemical hydroxylamine killed five people at the Concept Sciences plant outside Allentown, Pennsylvania. And in 2013, a fertilizer plant explosion killed a dozen people at West Fertilizer just north of Waco, Texas.