U.S. Steel on Friday pinned the blame for the deadly explosion Monday at the Clairton Coke Works on a valve failure that led to coke oven gas igniting, according to the company’s preliminary findings.

U.S. Steel says 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, sent 10 to hospitals and damaged part of the sprawling facility.

U.S. Steel says it believes a gas valve failure led to the explosion Monday morning at its Clairton Coke Works, in which two workers died and 10 others were injured.