A small corroded pipe caused the initial blast at a Chevron refinery in Richmond, Calif., this past August, but the oil giant’s own firefighters, in their haste, may well have been responsible for the fire’s spread.

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The San Francisco Chronicle reports on the ongoing investigation, which reveals that early efforts to put out the flames may have actually stoked them.

“One theory we are exploring is that emergency response activities inadvertently accelerated the rate of the leak,” said Daniel Horowitz, managing director of the Chemical Safety Board. “We are comparing possible tool marks on the pipe with tools recovered from the incident.”

One tool that may have inflicted the apparent damage is a Halligan bar, which has a hook-like implement with a sharp end. Firefighters are commonly equipped with the device to help them gain entry into burning buildings.