People view the fire at the Chevron Richmond Refinery in Richmond, Calif., Monday, Aug. 6, 2012. The fire, first reported at 6:40 p.m. Monday, is burning in a process unit at the refinery, officials said. It was sending smoke over the cities of Richmond and San Pablo. (AP Photo/San Francisco Chronicle, Lance Iversen) NORTHERN CALIFORNIA MANDATORY CREDIT PHOTOG & CHRONICLE; MAGS OUT; NO SALES

RICHMOND, Calif. (AP) - Investigators probing the cause of a blaze at Chevron's Richmond refinery are looking at heaters and responding emergency vehicles as possible ignition sources for the massive vapor cloud that spewed from an old, leaky pipe.

U.S. Chemical Safety Board investigator Don Holmstrom says refineries like Chevron's have fired heaters that could have provided a spark for the Aug. 6 inferno.

Holmstrom said a responding Chevron fire truck could be another possible source of ignition, since the 150-to-200-foot-high vapor cloud covered a large area.

The blaze knocked an important refinery unit offline, reducing the facility's production and sending thousands of people to hospitals with breathing and eye irritation complaints.

The average price for a gallon of regular on Wednesday in California was $4.09, up from $3.86 Aug. 7.

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