One person was killed and two were unaccounted for after a huge explosion at a Pennsylvania steel coking plant, per officials.

Emergency crews rushed to the US Steel Clairton Coke Works plant Monday morning after locals heard a loud boom in the Pittsburgh-area facility.

At least nine people were taken to hospitals and were being treated for unspecified injuries.

Officials previously told WTAE there was a rescue mission underway for people trapped under rubble in the plant. 

The Allegheny County Emergency Services said a fire at the plant started around 10.51am EST and that five people have been taken to hospitals. The agency did not provide any more details on those people transported and would only say it was an ‘active scene.’

Footage from the scene showed firefighters battling flames at the plant as ambulances go in and out of the area. 

The number and extent of injuries remained unclear but multiple medical helicopters were seen on their way to the plant.

Officials declared a mass casualty incident to allow for additional EMS resources to respond. 

‘We had dozens of walk-ins and wounded. There were some transports. I’m still working on getting all the details,’ said Kasey Reigner, a spokesperson for Allegheny County’s emergency services.

Clairton Mayor Rich Lattanzi issued a statement on X, writing:  ‘Just a report of an explosion! Asking for multiple ambulances! My prayers go out to anyone involved in this tragedy and their families and community.’

He later said in a post on Facebook: ‘It was a terrible incident and our prayers go out to all the families affected by this. There are people hurt and that is the utmost importance.’

The Mayor added that the ‘issue and the affected equipment and gas are contained and not impeding on the Clairton Community or surrounding communities.’

Senator John Fetterman, who rushed to the scene, asked people to stay away from the area.

‘For those in the area, please listen to local officials and avoid the area,’ he said on X. ‘Keeping those injured and all who are impacted in my thoughts right now.’

Local Melanie Dawn wrote on Facebook: ‘This explosion made our houses shake. I thought my son had hit his gas tank on the dirt bike. 

‘My niece felt it on Chambers Street, I felt it on Shaw Avenue. My teenage son’s ears were ringing.’ 

In a statement, U.S. Steel said an ‘incident’ occurred at the plant’s coke oven batteries 13 and 14.

The company, now a subsidiary of Japan-based Nippon Steel Corp., said emergency teams were immediately dispatched to the scene, but it gave no other details about the cause of the explosions, casualties or damage.

The company’s CEO, David Burritt, said in the statement that U.S. Steel is working with authorities to investigate the cause.

Governor Josh Shapiro said on X his administration was in touch with local officials dealing with the aftermath of the explosion.

‘The scene is still active, and folks nearby should follow the direction of local authorities. Please join Lori and me in praying for the Clairton community,’ he said.

The plant, a massive industrial facility along the Monongahela River south of Pittsburgh, is considered the largest coking operation in North America and is one of four major US Steel plants in Pennsylvania that employ several thousand workers.

In recent years, the Clairton plant has been dogged by concerns about pollution. In 2019, it agreed to settle a 2017 lawsuit for $8.5 million. 

Under the settlement, the company agreed to spend $6.5 million to reduce soot emissions and noxious odors from the Clairton coke-making facility, on the Monongahela River about 20 miles  south of Pittsburgh.

The company also faced other lawsuits over pollution from the Clairton facility, including ones accusing the company of violating clean air laws after a December 2018 fire damaged the Clairton facility’s sulfur pollution controls.