“We owe them an answer for what happened,” Shapiro said of the workers.

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 2010 accident—which prompted multiple lawsuits against the company and left one worker with burns over 40 percent of his body and permanent disfigurement—is part of a troubling pattern at the 125-...