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.
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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.
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.