Fire and police officials say a large explosion occurred Wednesday morning at the Exxon Mobil refinery in Torrance.

As of about 10:30 a.m., residents in the area had been asked by police to shelter in place -- meaning they should remain indoors -- and turn off any heating and air systems and close their windows.

Police announced a "second-alarm fire explosion." Police and firefighters were on the scene, and Del Amo Boulevard was closed between Maple and Crenshaw.

Exxon Mobil spokesman Todd Spitler said the incident happened at 8:50 a.m. and that refinery workers activated "emergency procedures."

"Our main concern is for the safety of our employees and our neighbors," he said. "We are accounting for all personnel and still evaluating the cause of the incident, or the occurrence or amount of any damages."

The South Coast Air Quality Management District says inspectors have been sent to assess air quality following the explosion.

Residents took to Twitter on Wednesday morning reporting that there had been an explosion and that ash was falling from the sky.

Torrance Fire Capt. Steve Deuel said the incident could be a result of a petroleum product leak that prompted workers to activate the refinery's flare system.

Looks like damage caused by the explosion at the Torrance Exxon Refinery. #Torrance pic.twitter.com/dY1flY8d2e — Jennifer Thang (@jenniferthang) February 18, 2015

Thirteen schools in the area, including seven elementary schools, were sheltering in place, said Tammy Khan, a district spokeswoman. Outdoor activities were limited for students in schools close to the refinery on the north and west sides of the district.

Torrance Mayor Patrick Furey told KNBC-TV that the material in the air was not caustic: "There's not much wind today, so it's not dissipating that quickly," he said.

The refinery is at 3700 W. 190th St. in Torrance. It covers 750 acres and employes 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.

The refinery's flare system shoots flames into the sky and, according to Exxon Mobil, can create a rumbling noise that also can be felt. It is activated during so-called unplanned operational interruptions, according to the refinery and flare-ups occur when vapors, air and steam violently mix.

Weird ash like substance falling in Torrance! What's up?? pic.twitter.com/UjVcvDnhEA — PVReiki (@PVReiki) February 18, 2015

@KTLA what happened at the torrance refinery, the gourd shook and we heard an explosion. pic.twitter.com/iqnD6cLgIx — Bri (@Bri0149) February 18, 2015

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