Fire officials are investigating the cause of a fire at a commercial building being renovated by Costco in Fairbanks, authorities say. 

According to a release from the Fairbanks Fire Department, a Fairbanks police officer was driving past the building located on College Road on Friday and noticed black smoke coming out of the roof of the building. 

"The officer reported the situation to the Fairbanks Emergency Communication Center (FECC) and they dispatched Fairbanks Fire Department (FFD) at 10:06pm," FFD wrote. 

Fire crews and first responders arrived on scene at 10:14 p.m. FFD had the fire out in less than a half hour, the release said. 

"The damage was limited to a 20 by 20-foot area on the roof of the building," FFD said. 

Officials say there were construction workers at the scene when the fire started but no one noticed it until fire crews arrived. 

"It was reported that some welding took place earlier in the day underneath the area where the fire started," FFD said. 

The investigation is ongoing. 

Anyone with any information about the fire is asked to call Chief Styers at 450-6600.

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