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

• At least one person is dead, 10 injured and one unaccounted for in an explosion at the Clairton Coke Works in Pittsburgh, Pennsylvania. The cause of the explosion is unknown and officials are investigating, US Steel said.

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

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.

“It is with profound sadness that I share with you that the U. S. Steel colleague who had been unaccounted for has been located and confirmed deceased, following an extensive search and rescue effort," David Burritt, the company's CEO, said in a statement. At least 10 people were injured in the blast, officials said.

Authorities said their investigation has shifted to the cause of the explosion after they accounted for everyone who was missing.