Authorities say carbon monoxide and gas alarms were triggered shortly before a fatal explosion at US Steel 's Clairton Coke Works.
The U.S. Chemical Safety Board’s preliminary investigation reveals the blast occurred during maintenance on a gas isolation valve in the facility’s basement.
Gas alarms went off just before the explosion, prompting an evacuation order less than a minute before the blast.
The valve involved was old, manufactured in 1953, and had known issues.
The CSB is continuing its probe into the cause and safety practices at the plant.
U.S. Steel says it’s cooperating and prioritizing employee safety.
The CSB will issue a final report once the investigation is completed.