BANGOR, Maine — A vacant house at the corner of Union and Pond streets was gutted by fire early Wednesday morning.

The blaze at 324 Union St. broke out shortly after midnight, Bangor Assistant Fire Chief Tom Higgins said at the scene.

Fire was spewing through the roof of the two-story structure when firefighters arrived, Higgins said.

Higgins said the fire started at the back end of the building and spread quickly, but officials aren’t sure what caused it yet.

“We have the fire marshal’s office here this morning to start an investigation,” Higgins said.

Firefighters from Bangor, Brewer, Orono and Veazie worked throughout the night despite temperatures in the single digits and snow.

“Crews have worked very well in the cold and snow,” Higgins said. “We had a lot of water flying around all night.”

Higgins said it took about two hours to get the blaze under control.

Higgins said it’s unclear who owns the building or whether the fire was suspicious.

All hands were struck as crews signed off at the scene. A Brewer Engine Company was added to the all-hands as well. Special call units included: an Engine company from Orono, a second ladder (Ladder 6), the Heavy Rescue, and an engine (Engine 2) company called in.

Total response: 4 Bangor Engines, 3 Bangor Rescues, 2 Bangor Ladders, 2 Assistant Chiefs, Heavy Rescue and 2 mutual aid Engines. An rescue from Orono FD covered EMS, and a Veazie Engine covered Station 5.