In a preliminary report Wednesday, the National Transportation Safety Board said the 30-inch transmission pipes that burst were under higher pressure than usual, but not by much, the San Francisco Chronicle reported.
The explosion in San Bruno killed eight people.
The California Public Utilities Commission will decide whether Pacific Gas and Electric Co. can return the 46-mile pipeline to its normal pressure, the Chronicle said.
Richard Kuprewicz, a pipeline safety consultant in Redmond, Wash., said PG&E and the utilities commission "should be working together on how they are going to supply gas this winter. You don't raise the pressure when you don't yet know why the thing failed."
The federal safety board's preliminary report said the segment of the pipe that burst in San Bruno was of uniform thickness when measured after the blast.
"If you had significant internal corrosion, you would expect that wall thickness to be significantly decreased," Brigham McCown, a Texas attorney and former head of the federal Pipeline and Hazardous Materials Safety Administration, said.
McCown said small and isolated corrosion may yet turn out to be a factor in the pipes' failure.