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Preliminary Report on San Bruno Explosion

Justin Sullivan/Getty

Workers stand near the epicenter of the gas pipeline explosion in San Bruno.

The National Transportation Safety Board has released a preliminary report on last month's gas pipeline explosion in San Bruno. 

The report doesn't say why the pipeline blew up, but it provides new details on the incidents leading up to the disaster.

According to the report, PG&E was working on the power supply that fed electricity to a valve controlling gas flow in the pipeline in San Bruno. There was a power failure just before the accident, and the valve opened, increasing gas pressure in the pipeline. That finding is no smoking gun -- pressure in the line was still below the maximum allowable.

Some safety experts have suggested that corrosion in the pipeline could have weakened it, but the NTSB reports no evidence of that in the segments damaged by the explosion.

In a statement, PG&E's President Chris Johns thanked the NTSB for its report, and said the company will do everything it can to cooperate with the continuing investigation.

The final report from the NTSB won't be available for several more months.    

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