U.S. Steel released preliminary findings about the potential cause of Monday's deadly explosion at U.S. Steel Clairton Coke Works.

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.

“Pressure built inside the valve, leading to valve failure and coke oven gas filling the area and ultimately exploding when finding an ignition source,” U.S. Steel said.