During a press conference Monday, U. S. Steel chief manufacturing officer Scott Buckiso said the company was trying to determine what had happened, according to The Associated Press. 

The incident occurred around 10:47 a.m. at Clairton's Nos. 13 and 14 coke oven batteries. A first explosion was followed by two more secondary explosions, the Tribune-Review newspaper reported. Emergency personnel from two counties, 14 fire departments and 20 EMS companies responded, the Tribune-Review said. 

U. S. Steel employees also jumped into action, rescuing fellow workers and stabilizing the scene, Buckiso said. 

“I want to express my deep gratitude to our emergency response team, who acted immediately in the wake of this tragic incident, and to all who provided support on site," U.S. Steel president and chief executive officer David Burritt said in a statement. "I know that here in Clairton, and across our U. S. Steel family, people are hurting. Let’s continue to do what we do best — stand together and support one another through these hard times." 

Burritt said the cause is being thoroughly investigated. 

"I end every meeting and every message with the words, ‘Let’s get back to work safely.’ That commitment has never been more important, and we will honor it," he said.