MADISON, Wis. -- Madison fire investigators are trying to determine what sparked a house fire on the city's southwest side early Sunday morning. Firefighters found no one was inside the house.

Firefighters went to the home in the 6500 block of Appleglen Lane at about 1:27 a.m. after receiving a 911 call from an Alliant Energy worker. The worker was in the neighborhood responding to a power outage when he said that he saw the fire.

Firefighters said that when they arrived, smoke was coming from the home. Once inside the home, crews saw flames had burned through part of the floor, making it unsafe to continue operating there. They backed up and proceeded to a lower level where they could safely attack the fire from below the damaged floor.

Other crews searched the home and confirmed that there were no occupants inside. The fire was declared under control at 1:53 a.m., officials said.

The fire caused an estimated $175,000 in damage and has extensive fire and smoke damage. Fire investigators are searching for the cause of the fire, WISC-TV reported.