TORRANCE, Calif. (AP) — An explosion devastated part of a major refinery on Wednesday morning, raining down ash in the area and, experts say, likely helping to increase California gas prices, which have been creeping up in recent weeks.

Gas prices in California have been inching up even before the Exxon Mobil accident. It’s the second refinery in the state to have service interrupted. A Tesoro refinery in Northern California that was running at half speed was shut down earlier this month after the contract with the United Steelworkers union expired.

Experts said California residents can expect to pay more at the pump.

But “you’re not going to see another gasoline apocalypse” like after the 2012 Chevron refinery fire that sent gas prices soaring over $4 a gallon, said Tom Kloza, chief oil analyst at the Oil Price Information Service.

As for the incident, a huge smokestack flare — in which workers were burning off flammable product after the explosion — could be seen for miles around. Four contractors suffered minor injuries as workers fled the site of the blast, according to Exxon Mobil Corp., which owns the refinery.

The facility, a structure several stories tall, was shattered. Crews poured water onto the structure afterward, and a fire spokesman said at midday the situation was controlled.

The blast happened in a recently installed processing facility, and the material involved was gasoline, Fire Department spokesman Steve Deuel said. The facility’s flare system was triggered to burn off fuel that could add to the fire, Deuel said.

Residents within a mile or two reported feeling a sharp jolt that they initially thought was an earthquake.

Electrical contractor Cory Milsap-Harris, 21, was in a switch house next door to the blast site keeping an eye on three colleagues working 8 feet underground in a manhole. “Everything was going smooth. Next thing I hear sounded like heavy metal next door. There was a loud bang,” he said. “You could feel the building shake a little.”

The blast reverberated in his ears despite the several layers of hearing protection he routinely wears, Milsap-Harris said. He rushed his co-workers outside, where people were running away from flames and black smoke.

Brittney Davis, whose office is about a block from the refinery, says the blast sounded and felt like something had rammed her building.

“The whole building shook. We couldn’t figure out what it was, but we stepped outside the door and the flames were shooting up from the refinery,” Davis said. “I could feel the heat from the flame.”

The refinery about 20 miles south of downtown Los Angeles covers 750 acres, employs over a thousand people, and processes an average of 155,000 barrels of crude oil per day and produces 1.8 billion gallons of gasoline per year, according to Exxon Mobil.

The four contractors were taken to a hospital for evaluation, company spokesman Todd Spitler said. The refinery sent many workers home for the day.

Spitler said the cause of the explosion was under investigation, and company officials were working with local agencies. Nearby roads were closed after the blast hit, shortly before 9 a.m., Torrance police Sgt. Paul Kranke said.

Students at 13 nearby schools were kept indoors, said Tammy Khan of the Torrance Unified School District.

The state Division of Occupational Safety and Health is leading the investigation, which can take six months to complete.

Cal-OSHA inspectors shut down a fluid catalytic cracking unit — a device used in refining oil — where the accident was thought to have occurred, said Erika Monterroza, a spokeswoman for the California Department of Industrial Relations.

The South Coast Air Quality Management District issued a smoke advisory for areas surrounding the refinery, and it dispatched inspectors to the facility with air sampling equipment. A representative from the U.S. Environmental Protection Agency was also on scene. Exxon Mobil said no harmful emissions were detected by its air quality monitors on the refinery fence line.

Gas prices in California have been inching up even before the Exxon Mobil accident. It’s the second refinery in the state to have service interrupted. A Tesoro refinery in Northern California that was running at half speed was shut down earlier this month after the contract with the United Steelworkers union expired.

Experts said California residents can expect to pay more at the pump.

But “you’re not going to see another gasoline apocalypse” like after the 2012 Chevron refinery fire that sent gas prices soaring over $4 a gallon, said Tom Kloza, chief oil analyst at the Oil Price Information Service.