WORCESTER — Investigators are trying to determine the cause of a three-alarm blaze that broke out Monday at the construction site of the new Doherty Memorial High School.

The school, on Highland Street next to the current, 56-year-old Doherty High, is about two years away from completion.

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The existing Doherty High next door will open as planned Tuesday, according to Worcester Schools Superintendent Rachel H. Monárrez.

The fire — reported shortly after 3 p.m. while sending a massive plume of black smoke over the city that could be seen for miles — was mostly knocked down by crews in about 30 minutes.

“On arrival there was heavy smoke and flames from the roof of one of the buildings that is under construction,” Acting Deputy Chief Adam J. Roche said during a news conference at nearby Elm Park. “We immediately started a defensive attack on the fire, using aerial apparatus to put the fire out.”

A construction worker was transported to a local hospital with non-serious injuries, Roche said.

Roach said roofing material on fire was responsible for sending the thick, dark smoke into the air. The fire was atop metal decking, about five floors up.

“What I have been told is that it was some roofing insulation that was out upon the roof this (Monday) afternoon,” Roche said. “That’s obviously still under investigation.”

Roche said the concern now is the structural integrity of the roof. He said there was “some structural damage” to the roof but they need to investigate it further to figure out the extent of it.

Roche said the nature of the smoke originated from the insulation burning. He said the heavy smoke has all dissipated after the fire was put out and is not a health concern of neighbors.

He said the fire department experienced some “issues” with water hydrants.

Although foam roofing insulation can catch on fire and is very flammable, Roche said it’s safe to put in a building when you install the sprinklers in. The building is one of four buildings under construction, Roche said.

Roche said there are standpipes connected to the building but that they are currently dry because there is no water to the building.

“We typically require standpipes as the floors go up to the floor below,” Roche said. “We don’t have water to the building yet. We could have laid the hydrant lines into standpipes but due to the proximity of the standpipes to the roof, the fire, they would not have been usable that way.”

Speaking at a news conference, Monárrez praised first responders and said she was thankful that students from nearby Doherty were safe.

The cause of the fire is under investigation. The structural integrity of the building will be assessed in the coming days.

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