CLAIRTON, Pa. —

The U.S. Chemical Safety and Hazard Investigation Board provided an update Monday on its investigation into the fatal U.S. Steel Clairton Coke Works facility explosion that occurred last month.

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According to a release, preliminary information "indicates that the explosion occurred during maintenance activities involving a gas isolation valve in the basement of the Battery 13/14 transfer area at the facility."

U.S. Steel employees, who were assisted by MPW Industrial Services employees, were said to have been performing a procedure to close and reopen the valve. When they attempted to flush the valve seat with water, gas monitors signaled

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