BANGOR – A fire charred the second floor of a building on outer Union Street Friday night.

No one was injured in the fire as the building was unoccupied at the time.

A passer-by was believed to have reported the fire to the Bangor Fire Department around 8 p.m. The first crew arrived a few minutes later, according to Assistant Fire Chief Rick Cheverie. He said the hydrants in the area have low water flow and an Air National Guard tanker was brought in to aid in extinguishing the fire.

Crews had the fire under control by 8:30 p.m., although not before the building had sustained significant damage. The rafters were burned to the point where they started to crack and Cheverie said there was still a possibility the roof could collapse during the night.

“The fire was so intense that it blistered the siding of the adjacent house, which is 35-40 feet away,” Cheverie said.

No cause had been identified Friday night, although Cheverie said a crew would come in to conduct an investigation first thing Saturday. He also said two men would probably stay on through the night to make sure the fire didn’t flare up again.

The second floor was “deeply charred,” Cheverie said, and the building suffered considerable smoke and water damage on both floors.

Cheverie said the building’s owner, Bruce Watson, had a wood- finishing shop on the second floor. Cheverie estimated the damage on the building to be between $20,000 and $25,000 and said the building was uninsured.

The Bangor Police Department assisted during the incident by diverting traffic from Union Street for almost an hour.