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.

Two workers died and at least 10 people were injured after multiple explosions at the U.S. Steel Clairton plant.

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.