Bangor firefighters spent an hour locating and dousing a fire at the old Waterworks on State Street late Monday night.

Fog, smoke and darkness complicated efforts to put out the fire, which began in the basement of the brick building. Smoke filled the building and billowed out windows that were opened by firefighters. The fire was called in shortly after 10 p.m. and was mostly extinguished shortly after 11 p.m., according to Assistant Fire Chief Frank Dinsmore.

Dinsmore said the fire was difficult to locate because of the darkness, fog and smoke. He said firefighters would continue to search the building to see whether the fire had spread to another part of the riverside structure.

Although no flames were visible from outside the building, the fire was discovered by Bangor Police Officer Dan Herrick, who was passing by the Waterworks in his patrol car and noticed what looked like heavy fog. He also smelled the telltale odor of fire.

Bangor police prevented traffic from entering the Waterworks area, which is located east of Eastern Maine Medical Center on State Street.

Dinsmore said a fire marshal would be called in to investigate the origin of the fire. The extent of damage wasn’t immediately known.

And it was not known late Monday night how the fire might affect plans by Chafe Restaurant Corp. to locate a Muddy Rudder Restaurant in the old structure. Last year Chafe, which operates a Muddy Rudder eatery in Yarmouth, earned approval from the Bangor City Council for more than $1 million in city financing to go toward a $2.6 million project to turn the old hydroelectric facility into a restaurant.

The city has spent 20 years looking for a developer for the historic property, which was built in 1875 to provide a source of water for the city.