A section of the Richmond refinery erupted in flames on Aug. 6 and sent a cloud of gas and black smoke over residential areas. Thousands of people sought medical treatment.
The San Francisco Chronicle (http://bit.ly/VmiRkU) says a metallurgical report showed the 40-year-old pipe was already weakened by the heavy sulfur content of the crude oil being pumped through it.
The investigation has also found that the pipe appears to have been punctured from the outside.
Investigators are examining firefighter tools to determine if they accidentally damaged the line while tearing away insulation.
Information from: San Francisco Chronicle, http://www.sfgate.com