The fire erupted when a vapor cloud ignited last Monday. The blaze endangered more than a dozen workers in the immediate vicinity. The resulting thick, black smoke caused thousands to seek medical attention for related health issues. It was marked as one of the most serious U.S. refinery fires in recent years.
Investigators want to know why Chevron did not replace the pipe when it carried out a major inspection in November.
The Richmond refinery, located about 10 miles northeast of San Francisco, produces about 16 percent of California's daily gasoline supply. The fire is being blamed for a spike in local gas prices to well over $4 a gallon.
The Associated Press contributed to this report.
