BOSTON (CBS) – A fire ripped through the top floor of a building under construction on Dorchester Ave. Wednesday afternoon. Heavy flames broke out on the sixth floor of the Treadmark Building around 2:30 p.m.

Boston Firefighters battle large blaze on Dorchester Ave. (WBZ-TV)

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The fire quickly spread, forcing firefighters to attack it from the exterior only. More than 125 firefighters responded and crews remained at the scene overnight watching for hot spots.

Service at the Ashmont Red Line station was shut down due to thick smoke and residents in the neighborhood were told to stay inside due to the poor air quality.

Boston Fire Commissioner Joseph Finn says a “collapse zone” was established around the building as a precaution.

“The air conditioning units did collapse into the top floor,” Finn said. “They were buckling.”

It is too early to determine the cause of the fire, and crews will be unable to enter the building until at least Thursday afternoon.

Joe Greathead was among the construction workers who first spotted trouble, a little smoke on the sixth floor.

“At first it was just a little smoke out of the wall,” Greathead said. “Once everybody was out of the building and the fire department showed up and it started blazing, it was crazy.”

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The building, which is currently unoccupied, has 83 units. Finn says the sprinklers were not yet turned on and that contributed to the fire getting out of control. The fire alarm system was set to be tested on Thursday.

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The building is a mix of condos and apartments and was set to open in July.

There were 32 condos on the top two floors of the building which were already sold. Almost all of the apartments on the lower floors were all rented out, and a business was set to move in on the first floor.

Boston Firefighters battle large blaze on Dorchester Ave. (WBZ-TV)

Commissioner Finn says the building is not a total loss and the developer plans to rebuild as soon as possible.

“We care deeply about the Ashmont neighborhood, and are heart-broken,” said Jim Keefe of Trinity Financial. “The Boston Fire Department is working tirelessly and we are so grateful for their efforts. We are fully committed to making sure this project gets rebuilt as quickly as possible.”

The building cost $45 million in total development.

Foam fills Dorchester Ave. as firefighters battle blaze (WBZ-TV)

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No serious injuries have been reported. One construction worker was transported to the hospital for chest pain during the initial part of the fire.