Investigation Ongoing in Fire at Complex Under Construction in Boston

Officials are launching an investigation into the cause of a massive fire that broke out in a six-story building under construction in Boston's Dorchester neighborhood and where firefighters are still putting out hot spots nearly 24 hours later.

The fire broke out at the 1971-1977 Dorchester Ave. construction site, just outside Peabody Square, around 2:30 p.m. Wednesday.

"This was a difficult fire for us," Boston Fire Commissioner Joseph Finn said.

The sprinkler system at the 83-unit Treadmark building did not go off. Investigators and Finn are asking why.

"Did it fail?" Finn asked at a press conference Thursday. "Was it offline or was it shut down?"

The construction is what's called "stick-built," a technique that uses two-by-fours instead of larger wood or steal beams for taller buildings. Finn says the method is a major concern for firefighters.

"If you're lighting a fire in the fireplace," he explained, "the smaller stuff lights first. The larger stuff takes time to burn."

The top floor of the upscale building, where investigators believe the fire started, was a major concern for firefighters. At one point, firefighters were told to get off the roof as the building weakened and air conditioning units collapsed into the roof.

Finn added the building's sprinkler system was offline, and why it wasn't on is one point of the ongoing investigation.

"What probably contributed to [the fire] was that the sprinkler system was offline, which was a major contributing factor and hopefully the investigation will determine why it was offline," he said.

The Treadmark building was scheduled for inspection on Thursday before opening its doors in a few days.

"All of the permits and inspections were totally up to par," Inspectional Services Commissioner William "Buddy" Christopher said.

Finn later told the NBC Boston Investigators it's unclear if fire inspectors have ever been in the complex. He says it is incumbent upon the building's owner to make sure the sprinkler system works, but they don't have to prove it does until the final walkthrough with the fire department.

Tenants had expected to move in sometime mid-July. City officials will work with the displaced families, according to Mayor Marty Walsh.

Dorchester Avenue has been closed from Ashmont Street to Washington Street. MBTA service has resumed at Ashmont Station, which was shut down Wednesday night.