The deadly explosion at Clairton Coke Works near Pittsburgh occurred when a gas valve was flushed before planned maintenance, U.S. Steel said in preliminary findings.

Two people are dead and 10 others injured after an explosion at U.S. Steel’s Clairton Coke Works near Pittsburgh. The recovery operation has ended, and the cause is under investigation.

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.