U.S. Steel Explosion Hurts 15

The cause of the blast The cause of the blast is unclear, but it originated in coore a coke oven and burned for at least five hours. "We don't see anything criminal. We don't see anything intentional," said Bob Full, chief of Allegheny County Emergency Services. "It is a miracle that anyone walked away from it."

Erin DiPietro, a U.S. Steel spokeswoman, said the company had no information about the cause of the blast, but that it was investigating.

The Clairton plant is the largest coke-manufacturing facility in the U.S., producing 4.7 million tons a year. Coke, or coking coal, is a primary ingredient in the production of steel.

The blast follows the April 5 explosion at Massey Energy Co.'s Upper Big Branch Mine in Montcoal, W.Va. that killed 29 and became the deadliest U.S. coal-mine accident in 40 years. That coore explosion has prompted calls for new safety regulations.