An explosion at a U.S. Steel plant near Pittsburgh left two dead and sent at least 10 to hospitals Monday and heavily damaged the sprawling facility.

Axar.az informs, citing AP, one worker was found alive in the wreckage hours after the explosion sent black smoke spiraling into the midday sky in the Mon Valley, a region of the state synonymous with steel for more than a century. Allegheny County Emergency Services said a fire at the plant started around 10:51 a.m.

The explosion, followed by several smaller blasts, could be felt in the nearby community and prompted county officials to warn residents to stay away from the scene so emergency workers could respond.

At a news conference, Scott Buckiso, U.S. Steel’s chief manufacturing officer, did not give details about the damage or casualties, and said they were still trying to determine what happened. U.S. Steel employees “did a great job” of going in and rescuing workers, shutting down gases and making sure the site was stable.

Buckiso said the company, now a subsidiary of Japan-based Nippon Steel Corp., is working with authorities

U.S. Steel CEO David B. Burritt said the company would thoroughly investigate the cause.