The results were handed over to the National Transportation Safety Board (NTSB) in Washington on Tuesday.
The board chairman says Pacific Gas & Electric is to blame for a litany of failures leading up to the explosion.
The five-member NTSB voted that substandard welds and other problems dating to the 1956 installation of a PG&E's gas transmission line beneath San Bruno were the direct cause of the accident.
The utility is also being accused of exploiting weak safety oversight by state and federal regulators.
"It was not a question of if this pipeline would burst," said NTSB Chairman Deborah Hersman. "It was a question of when."
PG&E has said it didn't know of the welds because its records incorrectly listed the pipe as seamless. The utility also has been unable to produce other key records regarding the pipeline.
The September explosion killed eight people, injured dozens of others, and destroyed or damaged 55 homes. It was the worst pipeline accident in a decade.
The Associated Press contributed to this story.