Mankato Public Safety was called to a structure fire on Fourth Avenue early Sunday morning. Authorities say an outbuilding was on fire and a nearby building was beginning to burn when fire firefighters arrived on the scene around five-15 a.m. Mankato Public Safety says firefighters were able to quickly extinguish the blaze. The cause of the fire remains under investigation. Authorities say no one was injured and they estimate the damage at 15-thousand dollars.