Atlanta Fire Department spokeswoman Marian McDaniel confirmed the cause of Monday's blaze to The Atlanta Journal-Constitution.
McDaniel told the newspaper the building, which was undergoing repairs, did not sustain any structural damage, and said no injuries were reported.
No criminal charges were pending.
The exterior of a building in the 30-acre complex was heavily damaged by fire in May, as well. It was unclear whether both incidents happened at the same building.