More than a month after the deadly U.S. Steel Clairton plant explosion , investigators are revealing new insight into how it happened.

According to preliminary findings from the U.S. Chemical Safety Board, a gas isolation cast-iron valve was the source of the deadly tragedy.

"The normal phase of the valve is, and they were going to close the valve and then reopen it." Bill Steiner, from the U.S. Chemical Safety Board, said. "The exercising of the valve that day was an activity in preparation for a scheduled job the following week."

Investigators say employees were working to close and reopen a gas isolation valve in the basement of the battery transfer area.

When they tried to flush the valve seat with water, gas monitors began alarming. Evacuations were ordered, and less than a minu

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