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

Ten people were hospitalized after the blast at a coke plant in Clairton, Pa., near Pittsburgh. One person who had been missing was found dead late on Monday.

After an explosion rocked a steel plant outside Pittsburgh on Monday, workers scrambled into the wreckage alongside firefighters. A cloud of black smoke rose above a charred pickup truck as rescuers sprayed the scene with streams of water.