PLYMOUTH, Wash. (AP) — Federal and state regulators say the main cause of an explosion in 2014 at a southeastern Washington natural gas facility was inadequate procedures that allowed oxygen to remain in the system.

A pressure vessel ruptured at the Williams Northwest Pipeline facility in Plymouth, Washington, on March 31, 2014, injuring six employees and causing tens of millions of dollars in damage.

In a report released Wednesday the U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration said the mix of oxygen and gas ignited. Other factors investigators cited were procedures that lacked details for employees to follow and procedures that didn't address the piping configuration and allowed oxygen to be trapped.

Two storage tanks hold up to 14 million gallons of liquid natural gas to meet peak winter demands on the pipeline that carries gas through Washington, Oregon, Idaho, Wyoming, Utah and Colorado.

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