Investigators are looking at how a seemingly insignificant leak at one of the country's biggest oil refineries turned into a fire that sent acrid smoke above the San Francisco Bay area and hundreds of people to hospitals with health complaints.
Officials said Tuesday that the leak started as a drip so small that Chevron did not think it posed an immediate danger to the residents.
State law requires Chevron to "immediately" notify the public of any gas leak, fire or oil spill.
The refinery's fire chief says at the time, there was nothing to advise.
Engineers stripped insulation on the leaky pipe to find the source, which released a vapor of a flammable substance similar to diesel. Hours later, officials scrambled to warn residents to stay inside.