The leak occurred at the Catalyst Refiners plant in Institute as workers were preparing to shut down at least part of the facility, Kanawha County Commission Emergency Management Director C.W. Sigman said.

A chemical gas reaction occurred at the plant involving nitric acid and another substance, Sigman said, speaking at a news briefing. He added that there was “a violent reaction of the chemicals and it instantaneously overreacted.”

“Starting or ending a chemical reaction are the most dangerous times,” Sigman said.

Among the injured were seven ambulance workers responding to the leak, officials said.

Other people were taken to the hospitals in private cars or even in one case a garbage truck, Sigman said.

One person was in critical condition, Kanawha County Commission President Ben Salango said.

Vandalia Health Charleston Area Medical Center, one of several hospitals in the area, was treating multiple patients, some brought by ambulance, while members of the community were arriving Wednesday afternoon asking to be checked, hospital spokesman Dale Witte said.

Witte said patients were experiencing respiratory symptoms including cough, shortness of breath, sore throat and itchy eyes. They were being evaluated in the emergency room.

WVU Medicine Thomas Memorial Hospital in South Charleston said in a statement it has cared for a dozen patients, including eight who arrived by personal vehicle and were not at the scene but were in the area at the time. It said none of the injuries were considered life-threatening.

A shelter-in-place order was issued for the surrounding area and lifted more than five hours later. Officials said all the deaths and injuries occurred on the plant site.

“You had to get really close to the facility to smell it,” Sigman said.

The leak required a large-scale decontamination operation in which people had to remove their clothes and be sprayed down, authorities said.

Catalyst Refiners works to remove silver from what remains of chemical processes and can find thousands of dollars of the precious metal just by vacuuming the floors in a plant’s offices, Sigmon said.

Ames Goldsmith Corp., the owner of Catalyst Refiners, said it is saddened by the deaths and its thoughts were with all those impacted and their families.

“This is an unfathomably difficult time,” company President Frank Barber said in a statement released at the news briefing. “Our thoughts and prayers are with our colleagues and their families.”

Ames Goldsmith promised to work with local, state and federal officials as they investigate what happened.

The plant is located near Institute, a community about 10 miles (16 kilometers) west of Charleston, the state capital. The plant is in an region known as West Virginia’s “chemical valley,” although many plants that lined the area along the Kanawha River and produced hazardous materials have closed or changed ownership in the past several decades.

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Jeffrey Collins contributed to this report from Columbia, South Carolina.