(WPXI) – U.S. Steel has released new details on what investigators believe caused the explosion at the Clairton Coke Works plant that killed two people and injured ten others. U.S. Steel said an investigation conducted by third-party experts has revealed that the explosion happened while crews were flushing a gas valve in preparation for planned maintenance. Investigators learned pressure was building inside the valve, which caused it to fail. The failure caused coke oven gas to fill the area, which led to the explosion after it found a source of ignition. U.S. Steel CEO David Burritt said the plant is now safe to operate, but investigators continue to review surveillance video and interview employees as they work to find answers.