US regulators have ruled out a number of possible causes of a fatal Pacific Gas & Electric (PG&E) gas transmission line explosion which occurred on Sep. 9. Investigators said "the outer surfaces of the ruptured pipe pieces revealed no evidence of external corrosion." "No dents, gouges, or other physical indications consistent with excavation damage were observed. Additionally, no physical evidence suggests that a pre-existing leak occurred in the ruptured pipe pieces," the National Transportation Safety Board (NTSB) said in a summary of its conclusions.
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