Crews responded to a hazmat incident in Chelsea, Massachusetts, Tuesday morning, according to the Chelsea Fire Department.
The scene is located at 340 Marginal St., a Massachusetts Water Resources Authority building, which a spokesperson tells NBC Boston is under construction.
A gas alarm sounded around 4 a.m. and an operator went down under the three-story building.
After detecting hydrogen cyanide levels, power was shut off ahead of firefighters going inside.
The facility takes sewage from the area on the way to Deer Island.
Twenty-eight workers were inside and had to be evacuated. The building is now being ventilated.
Chelsea Fire said a firefighter was taken to a local hospital for a stress-related injury suffered while responding to the situation. It is not believed to be serious.
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Traffic has been rerouted from Marginal Street, and Chelsea's police chief is asking drivers to use a different route.
No further information was immediately available.