Feb. 26 (UPI) -- Firefighters battled a large fire at a Los Angeles-area oil refinery Wednesday, which closed a major Southern California freeway and blackened the sky with smoke.

The fire began late Tuesday night at the Marathon Petroleum oil refinery in Carson, Calif., and was knocked down about 5 hours later, around 3:50 a.m. Wednesday.


Large flames could be seen for miles away from the plant. Authorities weren't immediately sure what caused the blaze.

The California Highway Patrol closed Interstate 405 for about an hour near the plant, which is located about 5 miles northwest of Long Beach and 15 miles south of downtown Los Angeles.

Interstate 405, also known as the San Diego Freeway, spans about 70 miles from San Fernando in the north to Irvine in the south and is a major bypass auxiliary route for Interstate 5.

Flammable substances complicated the progress of firefighters.

"Firefighters continue to make progress reducing the size of this fire while providing protection to exposures," the Los Angeles County Fire Department tweeted early Wednesday before containing the blaze. "Residual pressure from remaining flammable gas is still contributing to limited fire activity. Engineers continue work to isolate fuel sources."

Residents as far away as Glendale, 20 miles north of the plant, said they heard something that sounded like an explosion before the fire.

Marathon officials told fire authorities the refinery's perimeter monitors hadn't reported any harmful leaks into the atmosphere.

The South Coast Air Quality Management District said it collected samples at the time of the fire to test for toxins. Officials expected to have results Thursday, the Los Angeles Times reported.

District officials warned nearby residents that if they see or smell smoke, "limit your exposure by remaining indoors with windows and doors closed."

The Carson refinery is the largest on the West Coast, with a capacity of 363,000 barrels of crude oil per day. The plant makes gasoline and diesel fuel, along with distillates, petroleum coke, anode-grade coke, chemical-grade propylene, fuel-grade coke, heavy fuel oil and propane.

Marathon, headquartered in Ohio, operates 16 refineries nationwide.