It happened around 11:45 p.m. at Yankee Peddler and Pawn Shop.
The firefighters were alerted to the blaze by Norwalk Police Officers responding to a burglar alarm who noticed a strong odor of smoke in the area.
When the firefighters arrived, they quickly forced entry and found a small fire in a rear office of the building, the Norwalk Fire Department said.
A rear door to the structure was also forced open to allow the building to be ventilated. The building is fortified with roll-down steel doors with heavy duty locks.
Smoke and soot damage filled the store, with fire and heat damage confined to the rear office.
Firefighters remained on scene ventilating the building until 1 a.m.
Deputy Fire Marshal Kirk McDonald is leading the investigation of the cause of the fire.
Three engines, two trucks and a rescue-and-command car responded to the fire.
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