A multialarm fire destroyed a $45 million apartment building in Boston that was in its final stages of construction.

Boston Fire Commissioner Joseph Finn says the blaze at the 6-story, wood-framed Treadmark building started around 2 p.m. Wednesday. Finn says the fire appears to have started on the top floor, moving quickly downward. Finn says the building became unstable after air conditioning units on top buckled and collapsed.

The fire department says it was forced to fight the fire from the exterior of the building due to safety reasons.

Officials say one construction worker was hospitalized for chest pains, and several firefighters were hospitalized with exhaustion.

Officials say the building was constructed all within code, and the cause of the fire is still under investigation.