RICHMOND -- Chevron firefighters responding to a small Richmond refinery leak last summer may have accidentally punctured a main pipeline before a huge Aug. 6 blaze, federal investigators said.

A crude-oil unit of the Richmond refinery burned, sending a cloud of gas and black smoke over residential areas. Thousands of people sought medical treatment, complaining of eye irritation and problems breathing. The unit remains closed.

An interim metallurgical report showed the 40-year-old pipe that failed was initially weakened by the heavy sulfur content of the crude oil being pumped through it, U.S. Chemical Safety Board spokesman Sandy Gilmour wrote in an email to the Bay Area News Group.

After a small leak sent hydrocarbons into the air, a small flash fire was put out. But a larger gash in the pipe released a bigger cloud of flammable gas, leading to the larger conflagration, the San Francisco Chronicle reported Friday.

The federal investigation found the pipe appeared to have been punctured from the outside.

"The CSB is examining the possibility that emergency response activities inadvertently accelerated the rate of the leak," Gilmour said. "We are comparing possible tool marks on the pipe -- where metal appears to be punctured inward -- with a firefighter's Halligan pike recovered from the incident."

Investigators are examining the metal to determine if it matches the shape of the firefighting tool used to tear away insulation from the damaged pipe, he said.

"However, we hasten to add that the blaze very likely would have happened even without this apparent puncture, but this external damage could have been an aggravating factor," Gilmour said.

Sean Comey, a spokesman for San Ramon-based Chevron Corp., said in an email to The Associated Press that the company is working closely with all investigations and conducting its own probe.

The company still believes the crude unit damaged in the fire will be back online sometime during the first quarter of 2013.

The rebuilding was delayed after Richmond city officials looked to independent consultants to confirm Chevron was replacing the damaged equipment with the best possible technology. Chevron threatened to lay off more than 600 workers if it failed to get the needed permits for reconstruction. The city approved the project last month.

The Chemical Safety Board has not released the consultant's report, but a draft version was circulated for comment to Chevron, Cal-Osha and a Chevron employee union in late December. Anamet Inc. of Hayward, the metallurgical consultant, compiled the report.

Whatever caused the fire, the federal agency questioned continuing to run crude while investigating the leak.

"Regardless of the exact sequence of events, this incident emphasizes the importance of effective decision-making in shutting down the unit promptly in case of a leak of this nature," Gilmour said.

Staff writer Matthias Gafni contributed to this report. Contact him at mgafni@bayareanewsgroup.com or 925-952-5026. Follow him at Twitter.com/mgafni.