The leak occurred at the Catalyst Refiners facility in Institute, according to Kanawha County Emergency Management Director CW Sigman.
A violent chemical reaction resulted in a powerful chemical effect, Sigman said at a press conference.
Two people died in the accident on Wednesday, and another 19 people were injured.
The accident occurred during a shutdown of part of the factory. A large area around the factory, located just over a mile outside Charleston, the state capital, was cordoned off and several people in the vicinity of the accident were evacuated.
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