Investigators believe a diesel leak may have ignited the blaze that broke out in at least one processing unit at the Chevron refinery in Richmond, Calif., Monday night.

The blaze, which was reported at about 6:30 p.m., prompted an hours-long level 3 shelter in place.

Crews used nitrogen and steam to battle the flames, which were contained and the fire marshall recommended the shelter in place be lifted at about 10:30 p.m., according to NBC Bay Area.

Residents in Richmond, North Richmond and San Pablo were advised to go inside, close all windows and doors, turn off all heaters, air conditioners and fans. If they're not using the fireplace, residents should close fireplace dampers and vents, and cover cracks around the doors and windows with tape or damp towels, officials said. Residents are also asked to stay off the telephone, unless there is a life-threatening emergency.

Lisa Fasano of the Bay Area Air Quality Management District said there are five air quality inspectors headed to the scene to collect air samples both up- and downwind of the fire.

Chevron's own fire department responded to the scene with about 60 firefighters. Surrounding local departments and haz-mat teams are also on site assisting.

BART has closed down the Richmond line until further notice.

No evacuations or injuries have been reported. The cause of the blaze was not immediately known.

Monday's incident was the latest at the Northern California plant, which processes up to 240,000 barrels of crude oil daily and is among the largest in the country.

In 2006, there was an explosion at this plant that sent hundreds of people to the hospital. There were also incidents in 1999 and 2007.

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