TORRANCE (CBSLA.com) — The cause of an explosion at the ExxonMobil Refinery was under investigation Wednesday.

A three-alarm fire was reported around 8:50 a.m. at a processing unit at 3700 West 190th St. in Torrance, according to officials.

When crews arrived on scene they found evidence of an explosion and a small ground fire, which firefighters quickly extinguished.

The explosion caused a petroleum leak, which prompted a refinery worker to activate the plant’s flare system, according to Torrance Fire Capt. Steve Deuel.

“As part of the safety measures, they activated the flare towers,” he said. “That’s normal when they have to aspirate.”

According to authorities, four contractors were treated for minor injuries.

Witnesses reported seeing dust and ash falling from the sky.

“At the time a big white plume of smoke went up it felt like an earthquake, and then we looked out and saw the plume of smoke over the Mobil, and right now I am looking at two of the stacks that are flaming,” Debbie Hernandez, who works in Torrance, told KNX 1070 Newsradio.

Residents and approximately 30 schools in the Torrance School District were instructed to shelter in place. That advisory was lifted by police around 11:45 a.m.

Air Quality Management as well as HazMat crews were on site evaluating, Torrance police said.

“Our main concern is for the safety of our employees and our neighbors. We are accounting for all personnel and still evaluating the cause of the incident, or the occurrence or amount on any damages,” Todd Spitler from Exxon Mobil said in a statement.

Del Amo Boulevard was temporarily closed between Maple and Crenshaw.

Meanwhile, the area around the refinery felt the effects of the explosion and subsequent fire.

Businesses two miles away from the fire were experiencing ash falling from above, resulting in people trying to stay indoors.

“I heard a big jolt, and I felt like the house was shaking,” resident Mashwe Hla said. “Everybody was on the street, and then we saw the black smoke coming out, and we knew it was the refinery.”

Area schools remained sheltered in place, with students remaining indoors to avoid exposure to potentially unhealthy air.

“If she inhaled the ashes, and the air quality was worse, she would have a bad asthma attack,” one mother said of her daughter.

Dozens of children were pulled from school by their parents at North High School.

Residents gathered Wednesday night to voice concern.

“If that stuff is laying on cars and everything else, what’s it doing in our lungs,” one resident said.

David Campbell of United Steel Workers says he, too, has concern over the air being breathed in vicinity to the plant.

“It is toxic, but the amount of exposure is what determines the affect on your body,” Campbell said.