U.S. Steel said pressure built inside the valve, leading to its failure. This allowed coke oven gas to escape and explode when it found an ignition source.
Neighbors of U.S. Steel's Clairton Coke Works and its workers react to the explosion at the plant on Monday that killed two and sent 10 to the hospital.
The plant was operating as normal before the blast, with crews preparing for a planned maintenance job. The investigation into the deadly blast is continuing.
Ten plant workers required hospital treatment for injuries— five were listed in critical but stable condition, and five others were treated and released.
Environmental groups want Nippon to clean up U.S. Steel's Mon Valley plants, which have racked up $64 million in fines and settlements in recent years.