CALGARY -- No one was injured when a southeast Calgary strip mall burst into flame early Sunday, but it took a lot of firefighters to help put it out.

The Calgary Fire Department says it was called to the scene at just after 6 a.m.

Upon arrival, crews found the fire, which had started outside the building, had spread to the roof.

A third alarm was called to ensure enough firefighters were on hand to extinguish the blaze.

No one was inside the building at the time the fire broke out, but official say it caused a lot of damage.

"There is extensive damage to three of the businesses in the middle of the strip mall, with minor to no damage to the businesses on the ends," the CFD wrote in a statement.

Crews are working on determining the cause of the fire.