HARRISBURG, Pa. (AP) — 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 and sent 10 to hospitals, the company said Friday.

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.

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