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The cause of the gas pipeline explosion in San Bruno, CA, that killed eight people and destroyed 38 homes in September was an inferior weld, officials said.

The metallurgical report released federal transportation safety officials identified for the first time a particular seam weld as the cause of the rupture in the Sept. 9, 2010, explosion.

The weld was half as thick as it should have been, the report said. Experts said the flaw would have been exposed during high-pressure water testing inspections the utility, PG&E, had ruled out as expensive and burdensome.

The utility company’s records, found by investigators to be wrong, indicated the pipeline carrying the gas was seamless. Instead, the San Bruno site was part of a cobbled assortment of potentially inferior 4-foot seamed pipes of possibly unknown origin.

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The pipe developed a crack “consistent with ductile overstress from the root of the weld,” according to the National Transportation Safety Board report.

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