FILE PHOTO: U.S. Steel's Clairton Coke Works plant is seen after an explosion in Clairton, Pennsylvania, U.S. August 11, 2025 in this screengrab obtained from an aerial video. ABC Affiliate WTAE via REUTERS/File Photo

(Reuters) -United States Steel said on Friday a fatal blast that killed two and injured 10 others at its Clairton Coke Works plant near Pittsburgh happened when flushing a gas valve, ahead of scheduled maintenance.

"Pressure built inside the valve, leading to valve failure and coke oven gas filling the area and ultimately exploding when finding an ignition source," the company said.

U.S. Steel added that the investigation into Monday's explosion is still in early stages and it will provide more information when possible.

The Clairton Coke Works is the largest coke manufacturing facility in the United States, employing about 1,300 workers. It operates 10 coke oven batteries, which produce about 4.3 million tons of coke a year.

(Reporting by Abhinav Parmar and Jaspreet Singh in Bengaluru; Editing by Alan Barona)