A federal safety panel blamed a series of missteps by one of the nation's largest gas companies for the largest pipeline accident in a decade, a suburban San Francisco explosion that killed eight people and incinerated a neighborhood.
The panel also warned there was no certainty that the problems that led to last year's blast don't exist elsewhere.
The National Transportation Safety Board wrapped up its investigation of the Sept. 9, 2010, accident in San Bruno, Calif., voting 5-0 Tuesday to approve a finding that actions by Pacific Gas & Electric were the probable causes of the explosion.
Lax regulations at the state and federal level also contributed to the accident, the board said.
"The aging pipelines, our oldest pipelines, really are without a safety net," board chairwoman Deborah Hersman said.
The board found that substandard welds and other problems dating to the 1956 installation of a PG&E gas transmission line beneath San Bruno were the direct cause of the accident. It also said the company's inadequate inspection program for pipelines, which allowed the bad welds and other weaknesses to go undetected, helped cause the accident.
The board also cited a lack of federal or state requirements for testing for older pipelines to detect defects.
It faulted the California Public Utilities Commission for failing to detect widespread internal problems with PG&E's safety procedures, including no automatic gas shut-off valves and shortcomings in its emergency response plan that contributed to the protracted duration of the incident.
"It was not a question of 'if this pipeline would burst,' " Hersman said. "It was a question of when."
The board made safety recommendations to regulators and the gas industry.
Among problems the board cited were a failure to learn from past accidents, the loss of key personnel after the company's 2001 bankruptcy, poor internal communications and bad morale among company employees.
PG&E President Chris Johns said the company has spent the past year making fundamental changes to its operations and management that will "put the safety of the public, our customers and our employees first."
About half the gas transmission lines in the U.S. -- roughly 150,000 miles -- were built before 1970 and are exempt from many federal safety requirements, safety board officials said.
"What we've seen in PG&E is hopefully the worst-case scenario," Hersman said.
The board was especially critical of PG&E's failure to provide investigators with critical records related to the source and installation of the section of the transmission line that ruptured.
Company records that were provided were also inaccurate on key points. They indicated the pipe that ran through San Bruno was seamless. But a laboratory examination later showed that the pipe was constructed with seam-welds that failed during the accident.
Also, company records showed the pipeline was of a uniform thickness in the San Bruno area. It was later found to be variable.
Such information is considered crucial to determining how much internal pressure a pipeline can be operated under, investigators said.
PG&E had set a maximum allowable pressure on the line of 400 pounds per square inch. It failed after reaching 396 pounds-per-square-inch in the hour before the explosion.