Chevron firefighters responding to a small California refinery leak last summer may have accidentally punctured a main pipeline before a massive blaze, federal investigators said.

A crude oil unit of the Richmond refinery burned on Aug. 6 and sent 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.

A 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, the San Francisco Chronicle reported.

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 newspaper reported Friday.

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

Investigators with the U.S. Chemical Safety Board are examining firefighter tools to determine if the line was accidentally damaged while they tore away insulation.

Investigators believe the fire could have started regardless of whether firefighters worsened the leak, but the tool damage could have been an aggravating factor.

Sean Comey, a spokesman for San Ramon, Calif.-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 it 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 that 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.