A vapor cloud ignited, sparking a blaze that forced the Aug. 6 closure of one of the units at Chevron's oil refinery in Richmond, Calif. The refinery processes oil into hydrocarbon products, including gasoline.
Don Holmstrom, lead investigator for the U.S. Chemical Safety Board, expressed concern that more than 20 workers were near the unit when the vapor cloud formed. Several people escaped less than a minute before the explosion.
"When you have a leak, you want to have control of the area," he was quoted by the San Francisco Chronicle as saying. "We are examining the issue of the number of people and why they were there."
Investigators have focused on an 8-inch line that was identified as corroded during a November 2011 inspection. Refinery workers were on site examining the situation for about two hours before the vapor cloud formed and later ignited.
CSB Chairman Rafael Moure-Eraso said the incident was a "close call."