FOND DU LAC, WI (WTAQ) - A restaurant in Fond du Lac is damaged by fire Friday.

Fire crews get the call just after 6 a.m. to the McDonald's at 770 West Johnson Street. Upon arrival, they saw heavy black smoke coming from a roof vent.

Everyone inside the restaurant managed to get out safely.

The blaze was contained to the deep fryer area and was quickly put out. However, firefighters say the area was extensively damaged.

Firefighters are still trying to figure out how the fire started. They say they are working with the building owner to make sure the fire suppression system will be working.