WASHINGTON (AP) — A federal safety panel unanimously concluded Tuesday that a series of actions by one of the nation’s largest gas companies caused a gas explosion last year that killed eight people and incinerated a suburban neighborhood near San Francisco.

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The five-member National Transportation Safety Board voted that substandard welds and other problems dating to the 1956 installation of a Pacific Gas & Electric Co.’s gas transmission line beneath San Bruno, Calif., were the direct cause of the accident. The board 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.

Contributing to the accident was lax oversight by state and federal regulators, the board said. Among the problems with government oversight was a lack of federal or state regulations on testing for older pipelines to detect defections, the board said.

The California Public Utilities Commission also failed to detect widespread internal problems with PG&E’s safety regime, including a lack of automatic gas shutoff valves and shortcomings in the company’s emergency response plan that contributed to the protracted duration of the accident, the board said.

“It was not a question of if this pipeline would burst,” said NTSB Chairman Deborah Hersman. “It was a question of when.”

The board, wrapping up a yearlong investigation, also made a series of safety recommendations to regulators and the gas industry before adjourning.

Similar systemic problems emerged following accidents on the company’s pipelines in 1981 in San Francisco and 2008 in Rancho Cordova, Calif. Yet, “PG&E missed earlier opportunities to make corrections that could have prevented the San Bruno tragedy,” said NTSB lead investigator Ravindra Chhatre.

The September explosion of a gas transmission line buried beneath San Bruno sent a giant plume of fire into the air that continued to burn for 95 minutes before PG&E employees were able to shut off the flow of gas. In addition to the eight deaths, dozens of people were injured and 55 homes destroyed or damaged. It was the worst pipeline accident in a decade.

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The board has made public 14,000 pages of information related to the accident. The investigation has raised questions about whether PG&E’s corporate culture put profits ahead of safety, contributing to the accident.

Investigators identified a substandard seam weld that went only halfway through the pipe wall at the rupture point. The 30-inch circumference pipe had other substandard welds as well. Evidence indicates the pipe was composed of several short welded pieces that didn’t meet any known specifications, investigators said.

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 pipeline also lacked automatic or remotely controlled shut-off valves. If valves on the line had closed immediately after the initial explosion, the gas-fed fire probably would have gone out in under 10 minutes, according to safety experts.

Coroner’s reports indicate at least five of the eight killed were trying to flee when they died. It is unclear based on information released so far whether the deaths of any of the five could have been avoided if different valves were in place, but experts said it was possible — and that others who were injured and homes that were leveled might have been spared.

Four years before the accident, PG&E officials rejected installing more automatic shutoff valves. A 2006 company memo said installing the valves would have “little or no effect on increasing human safety or protecting properties.”

A month after the San Bruno explosion, PG&E wrote California regulators that replacing or retrofitting the current 300 manual values in the company’s network with automatic or remote valves would cost $100,000 to $1.5 million per valve, depending on the difficulty of the installation.

Last week, federal regulators proposed forcing utilities to step up testing of older transmission lines and install more automatic shut-off valves.

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