The State Fire Marshal has determined the cause of Monday’s fire at a Quincy water treatment plant was a mechanical failure of an industrial circulation fan. 

The mechanical failure led to a release of a sulfur-based dust cloud, which exploded and lead to the ensuing fire. 

Five workers were injured and were taken to Boston Medical Center for treatment of respiratory issues and eye irritation. 

The area where the fire occurred is several stories underground. 

According to the Fire Marshal, workers were performing maintenance on an air handling system used to de-odorize gases and air prior to release into the atmosphere. 

Crews worked overnight Monday into Tuesday to extinguish the fire that injured five people on Monday at a waste water treatment plant in Quincy.

The treatment plant is part of the Massachusetts Water Resources Authority and is located on Nut Island, but the incident has no impact on the drinking water.

MWRA chief Fred Laskey said the incident happened several floors down in the building when crews were working with an air scrubber. 

"The goal is to let the firefighters fight the fire from the top down," Laskey said. "The issue is that the fire is inside duct work, so it’s very difficult; they can see it with the heat-sensing machine, but there’s still flames in there. And every time they find a way to get water into the duct, it simply pushes the fire farther into the duct."

A crane was brought in to aid the firefighter’s work. 

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