U.S. Steel says 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, sent 10 to hospitals and damaged part of the sprawling facility.

Nippon Steel (OTCPK:NPSCY) (OTCPK:NISTF) promised to invest $14B to keep US Steel's aging plants operating for decades, but this week's deadly explosion at the Clairton coking plant - the largest in the U.

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

An explosion at a U.S. Steel plant south of Pittsburgh killed two people and injured more than 10 others. The CEO has vowed to find out what caused Monday’s blast while the workers’ union says it will make sure there’s a thorough investigation.

Family members tell Pittsburgh's Action News 4 Steven Menefee, a father of two, was a loving man who would give you the shirt off his back.