The Richmond refinery, about 10 miles northeast of San Francisco, produces about 16 percent of California's daily gasoline supply.
Chevron said Saturday that it too was seeking to understand why the accident occurred.
"We agree that this is a serious incident that warrants thorough investigation," said Sean Comey, a company spokesman. "We are cooperating with all regulatory agencies and are committed to better understanding the root cause of this incident."
The inspectors have not yet seen testing records for the pipe that failed, but given its age and the condition of pipes connected to it, they believe corrosion is a strong possible cause of its failure. Investigators said, in general, that all pipes corrode over time.
The crude unit where the fire occurred is a key part of the refinery, helping to create a specialized blend of cleaner-burning gasoline that satisfies air-quality laws in California. On Saturday, the average price for a gallon of regular gas in California was $4.04, up from $3.86 cents Tuesday.
While high crude prices have driven prices up nationwide, the partial loss of production at Chevron's Richmond refinery has also had an effect on driving prices in the state even higher, analysts said.
The incident began Monday afternoon, when a small dripping leak was detected by refinery workers. When engineers responded to find the leak's cause, they removed insulation around the pipe.
"Due to the high temperature of the material in the tower, in excess of 600 degrees Fahrenheit, the gas-oil immediately formed a large flammable vapor cloud," chemical safety board investigators said in a statement.
Investigators said that more than a dozen Chevron refinery workers were engulfed by the vapor and narrowly escaped serious injury when it ignited.