HARRISBURG, Pa. (AP) — The fatal explosion last week at U.S. Steel’s Pittsburgh-area coal-processing plant has revived debate about its future just as the iconic American company was emerging from a long period of uncertainty.

The deadly explosion at Clairton Coke Works near Pittsburgh occurred when a gas valve was flushed before planned maintenance, U.S. Steel said in preliminary findings.

The mill, which is part of the recent merger between U.S. Steel and Nippon Steel, has faced scrutiny from federal and local regulators.