They were able to free one injured worker who was whisked to a hospital. But one more was still missing, and the area was too unstable to continue working, according to Matthew Brown, chief of Allegheny County Emergency Services.

A Pittsburgh-based crew from Pennsylvania Urban Search & Rescue was called in to help, some of whose members were already responding through their affiliations with local fire departments, Brown said. The team stabilized a wall at the plant and used an advanced camera to detect the missing employee. They pulled away the rubble and were able to extract the body of the worker, who died.

Monday’s explosion, which was powerful enough to shake nearby homes, killed two workers and injured more than 10 others. Five people ranging in age from 27 to 74 remained hospitalized Tuesday including the rescued worker, who was in critical but stable condition, according to the Allegheny County Police Department. Three were at UPMC Mercy, the region’s only level-one trauma and burn center.

The massive plant along the Monongahela River in Clairton converts coal to coke, a key component in the steelmaking process. The facility is considered the largest coking operation in North America and is one of four major U.S. Steel plants in Pennsylvania.

To make coke, coal is baked in special ovens for hours at high temperatures to remove impurities that could otherwise weaken steel. The process creates what’s known as coke gas — a lethal mix of methane, carbon dioxide and carbon monoxide.

U.S. Steel’s chief manufacturing officer, Scott Buckiso, said workers were conducting routine operations at the time of the accident.

Two loud booms that followed the initial blast were initially thought to be subsequent explosions, but Buckiso said they were from the activation of two relief pressure valves — a safety mechanism that operated as expected.

The cause of the explosion remained under investigation, and Democratic Gov. Josh Shapiro told reporters Tuesday that workers deserve an “answer for what happened.”

“We owe them the answers to their questions, and we owe them to never forget the sacrifices that occurred here yesterday,” Shapiro said. Before arriving at a news conference, he met with family members of a worker who died.

Allegheny County Executive Sara Innamorato told reporters she had assurances from U.S. Steel that it would continue to cooperate fully with investigators.

“We all share a common goal. We want to get to the bottom of what happened, and we want to prevent it from happening ever again,” Innamorato said.

Remembering the dead

Shapiro also used the news conference to pay tribute to the workers who died. The county medical examiner’s office identified one as Timothy Quinn, 39. and the other was not identified, with his family requesting privacy.

Shapiro described Quinn as a devoted father of three known to his friends as “TQ.” He served as a mentor and leader to other workers and was known for cracking jokes. A second-generation steelworker, he followed in his father’s footsteps and was a “mama’s boy” who, after working long shifts, would take care of his mother and look after his children and his girlfriend’s two children.

“His life was cut too short because of what happened here at this plant,” Shapiro said. “We have a responsibly to remember his legacy, to make sure his memory lives on. … We will make sure his children know his dad was a special man, a good man and a man who helped build this community with his hands the way his father did.”

CEO David Burritt called it an “extraordinarily difficult day” for a U.S. Steel family that has “suffered heartbreaking losses.” According to the company, the plant has approximately 1,400 workers.

The company, he said, is working closely with local, state and federal authorities. He would not speculate about the cause of the explosion.

“We will share as much as we can, as soon as we can, and we will take every step necessary to keep our people safe,” Burritt said.

Deanna Forkey was working behind the counter at the nearby Hometown Burgers & Deli, which her family owns, when she heard the explosion and the restaurant door popped open.

“When I looked out, all you could see was black smoke,” Forkey said. “Explosions over there aren’t really uncommon. We hear them a lot. But that one obviously was much worse.”

She said many plant workers are regulars at the restaurant.

“You start to build a little bit of a relationship,” Forkey said. “So it kind of pulls a little harder at the heartstrings.”

Plant has seen explosions in the past

In September 2009, a maintenance worker was killed in a blast. And in July 2010, another explosion injured 14 employees and six contractors.

According to online OSHA records of workplace fatalities, the last death at the plant was in 2014, when a worker was burned and died after falling into a trench.

After the 2010 explosion, the Occupational Safety and Health Administration fined U.S. Steel and a subcontractor $175,000 for safety violations. U.S. Steel appealed its citations and fines, which were later reduced under a settlement agreement.

A problem with a battery in February led to a “buildup of combustible material” that ignited, causing an audible boom, officials said. Two plant workers received first aid but were not seriously injured.

Richard Lattanzi, who retired from another nearby U.S. Steel plant and is now mayor of Clairton, recalled that the company once sought to ensure that workers “go home the way they came in.”

“Apparently it’s not happening right now, so we’ve got to get better,” Lattanzi said. “We have to rally around this plant right now.”

Pittsburgh attorney John Gismondi represented the widow of the worker who was killed in the 2009 explosion and three men who were badly burned in the 2010 blast. In a phone interview Tuesday, Gismondi said his immediate thought the previous day was, “Oh my God, not again at Clairton.”

Both lawsuits were settled out of court for “significant” amounts, he said.

“There’s a lot of gas on the premises. That’s fine, it’s part and parcel of what they do,” he said. “But gas is a dangerous substance, and you need to make sure safety protocols are being followed.”