(CNN) A grass fire was threatening an underground natural gas pipeline Thursday in the San Francisco Bay Area, forcing authorities to evacuate about 4,000 people and close an elementary school for the day, officials said.

The fire, which started Wednesday evening in the community of Bay Point, apparently was caused by an electrical power line that fell, according to Chevron Pipe Line Company, which operates the threatened pipeline.

At a news conference Thursday, the assistant Contra Costa County fire chief, Terence Carey, said crews had responded Wednesday evening and extinguished the vegetation fire, but were called back about an hour later for a fire in a Chevron vault with the gas pipeline running through it.

Chevron said it shut down the gas line and worked with firefighters to evacuate the area as a precaution.

About 4,000 people were evacuated from 1,400 homes, the Contra Costa County Fire Protection District said.

Relief workers deliver water and other supplies to a transit station parking lot that's serving as an evacuation point.

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