Preparations for a routine maintenance task may have led to an explosion at a U.S. Steel coal-processing plant near Pittsburgh that left two dead and sent 10 to hospitals, the company said Friday. U.S.

The company said pressure likely built up inside the valve, causing it to fail and leading to coke oven gas filling the area, which then exploded after being ignited.

United States Steel Corp. said Friday the fatal blast that killed two workers at its Clairton Coke Works facility in Pennsylvania happened when flushing a gas valve in preparation for planned maintenance.