Workers were heating coal and preparing for routine maintenance when an explosion rocked a U.S. Steel plant outside Pittsburgh

The Pennsylvania county where an explosion at a U.S. Steel plant south of Pittsburgh killed two people and injured more than 10 others announced Thursday that it is stepping up air quality monitoring in the area of the sprawling facility that has a troubled environmental record.

Authorities confirmed a second fatality after an explosion at the U.S. Steel Clairton Coke Works facility outside of Pittsburgh.