A natural gas pipeline burst Sept. 9, sparking a massive fireball that ripped through a San Bruno neighborhood. The explosion killed seven people and destroyed 37 homes.
The U.S. National Transportation Safety Board in a preliminary report found an electrical failure at a PG&E facility caused a pressure surge in the pipeline just before the San Bruno explosion. The report added that it took more than five hours for the company to shut off the gas after the explosion.
PG&E President Chris Johns said his company welcomed the preliminary report from the federal government.
"Although a final report and a conclusive set of findings are likely to be many months down the road, this initial release of information is an essential first step," he said in a statement.
Johns told a U.S. Senate panel on pipeline safety last month that additional safety and testing benchmarks warranted "additional discussion" by pipeline regulators.
PG&E compiled a list of 100 high-risk pipeline sections before the blast, though the San Bruno line wasn't on that list.