Federal investigators probing the cause of a massive Chevron oil refinery fire are focusing on possible corrosion in a decades-old pipe the company inspected late last year but did not replace.

Investigators with the U.S. Chemical Safety Board told The Associated Press on Saturday that the November inspection led Chevron to replace an old pipe connected to the one that failed Monday.

The fire exploded when a vapor cloud ignited, endangering more than a dozen workers in the immediate vicinity. The resulting blaze kicked up thick, black smoke and caused thousands to seek medical attention for related health issues in one of the most serious U.S. refinery fires in recent years.

The Richmond refinery, located about 10 miles northeast of San Francisco, produces about 16 percent of California's daily gasoline supply.

Chevron said Saturday it too is seeking to understand why the accident occurred.