Gasoline production at an Exxon Mobil Corp refinery near Los Angeles has been halted since an explosion there on Wednesday, a plant manager said on Friday, raising the possibility of higher fuel prices in parts of Southern California.

Investigators were trying to determine the cause of the blast in Torrance, California, south of downtown Los Angeles, which sent four contract workers to the hospital with minor injuries.

Refinery manager Brian Ablett said Exxon has an unspecified volume of gasoline in storage at the plant but did not say at what rate it might release those stocks, nor did he have an estimate for when production might resume.

California lacks pipelines to carry locally refined gasoline across the state and its supplies are further restricted by environmental rules requiring a less-polluting blend of gasoline and diesel fuel, which few out-of-state refineries make.

The explosion ripped open a massive, box-like filtration unit at the refinery, the electrostatic precipitator, and spread a layer of ash over nearby homes.

The blast also damaged a unit that cleans water for boilers and another unit that removes sulfur from gasoline, Ablett said. Moreover, inspectors need to assess damage to the fluid catalytic cracker, the facility's main gasoline-producing unit, he said.

"It will be some time before we fully know what happened," Ablett told reporters ahead of a meeting with local residents.

The California Department of Industrial Relations has opened a probe at the refinery, which normally can produce about 150,000 barrels of gasoline per day.

The refinery still has some production capacity, but the company's first priority is to make sure all the units are structurally safe, Ablett said.

The price of gasoline in Los Angeles surged to its highest level in about 18 months after the explosion on fears that local supply would tighten in the coming weeks.

Shorter supply could mean higher pump prices just as Californians and other U.S. motorists are benefiting from a slump in crude oil prices.

Exxon Mobil officials at a town hall meeting on Friday attended by more than 100 people faced pointed questions about ash from the explosion, which an aide to a local state legislator said was spread as far as 2 miles from the refinery.

The refinery's physician, Ellyn McIntosh, told the crowd an analysis of the ash shows it consisted of aluminum oxide, amorphous silica and a type of clay. The substance can cause irritation, but not significant health problems, she said.

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