The preliminary findings say how the company believes the explosion occurred. A union statement cautioned the investigation is in its early stages.

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.

The explosion killed two workers and hospitalized 10 with a blast so powerful that it took hours to find two missing workers beneath charred wreckage and rubble.