Published: Aug 11, 2012 12:31 PM EDT
Updated: Aug 12, 2012 6:30 AM EDT

SAN FRANCISCO (AP) - Federal investigators probing the cause of a massive Chevron oil refinery fire in Northern California are focusing on possible corrosion in a decades-old pipe the company inspected late last year.

Investigators with the U.S. Chemical Safety Board say they're trying to determine why Chevron didn't replace the pipe when it had a major inspection in November.

That inspection led Chevron to replace an old pipe connected to the one that failed in Richmond on Monday, catching fire, sending up black smoke and causing thousands to seek medical attention.

Also, investigators told The Associated Press on Saturday that more than a dozen refinery workers were engulfed by vapor and escaped serious injury by just seconds when it ignited.

Chevron says it too is seeking answers and is cooperating with the investigation.