At least one person dead after an explosion at a Pennsylvania steel work

UPDATE (2:58 PM): Officials provided an update on the deadly steel plant explosion in Pennsylvania Monday.

So far, one person is dead and 10 more are injured. Emergency services say it is still an active rescue mission, with one person still missing.

"We want to thank particularly the first responders who responded to the site, who showed up and ran towards danger. We're grateful for the leadership that they've shown, and we look forward to continuing to work in a collaborative manner, not just today, but in the weeks and months ahead, to ensure that an event like this doesn't happen again in the future."

Austin Davis, Lt. Governor of Pennsylvania

One person was already found in the explosion and has been sent to the hospital.

CLAIRTON, Pa. (NBC, KYMA/KECY) - At least one person is dead following an explosion at a Pennsylvania steel work.

Officials in the area say dozens were injured in the blast at the U.S. Steel Clairton Coke Works outside of Pittsburgh.

Crews are also searching for people who may be trapped in the rubble.

A mass casualty incident has been declared to allow for additional emergency resources from across the region to respond.

Local and state leaders say the explosion hits an industry that is key to the region.

"Well, it's a sad day for Clairton, and it's a sad day for the steel workers. These guys come to work and they put their gear on and they go to work to make a living for their family, not knowing if they're going to make it home or not. This is one of those situations. There's no guarantee in life, and I'm sad for everybody. I mean, these are my union brothers and sisters. I was a steelworker myself, and like I said, I'm heartbroken about it.

I just pray to God that everybody makes it and that the guys working in there aren't scared to come to work tomorrow. And I just ask everybody, Claire, please pray for the people in the mill, whether you like steel workers or you don't, pray for them and their family. Because today is this not a day that was designed."

Rich Lattanzi, Mayor of Clairton

"My whole thoughts go to the families of they've lost someone today, and it's just absolutely tragic, and now I am, I grieve for these families, and again, I'm proud to stand with the steel workers and to make sure why we do anything possible after this terrible tragedy.

Whether I was as a lieutenant governor when that happened, or as United States Senator, you know, the steel way of life here is sacred, and it's still dangerous, and my thoughts go only To the families that have might lose people today."

Sen. John Fetterman (D-PA)

An NBC affiliate in Pittsburgh spoke with a witness about the moment he heard the explosion.

"Heard this big boom, but I didn't think of nothing yet. I thought it was the train cars. You know, when you train backyards the tracks, all you hear is the trains, but then, I started hearing sirens. Came outside, and it was big, black smoke," said William Pearce, a witness.

The Health Department is also on-site to monitor the air quality, the spokesperson said in a statement.

Air quality monitors have not detected levels above federal standards, the department said, but residents who live within a one-mile radius of the plant are advised to remain indoors, close all windows, and "set HVAC systems to recirculate" out of an abundance of caution.

A cause for the explosion is unclear.

Governor Josh Shapiro says his administration is in touch with officials in the area.