Aug 15 (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.