Four workers suffered minor injuries Wednesday morning at the Exxon Mobil refinery in Torrance, California when a large explosion rocked the facility, causing a fire and sending ash raining down from the sky.

The Torrance Fire Dept. responded to the incident and extinguished what was described as a ground fire possibly fueled by gasoline.

The blast occurred around 8:50 a.m. Exxon Mobile officials said all employees were accounted for just after 11 a.m. Three of the four injured workers were treated at a local hospital and released. One was admitted for evaluation.

Although the facility continued to operate during the incident, Cal/OSHA – which has launched an investigation – ordered operations shut down at the unit where the explosion occurred until its safety can be verified, according to an agency spokesperson.

News sources said nearby residents and students and staff at 14 area schools in the area were ordered to shelter in place due to concerns about air quality. That order was subsequently lifted, but a smoke advisory was issued by the South Coast Air Quality Management District.

The Torrance refinery has a history of problems. In 1994, 28 workers were injured in an explosion. Another blast at the facility, in 1988, killed one person and injured nine others – and resulted in a criminal investigation.

The refinery covers 750 acres and employs about 650 employees and 550 contractors. The refinery processes an average of 155,000 barrels of crude oil per day and produces 1.8 billion gallons of gasoline a year.