Update 5:20 a.m.
A restroom exhaust fan was determined to be the cause of the fire, officials said in a press release.
The fire at the Fuddruckers restaurant on Main Street was reported at about 1:38. It caused an estimated $30,000 in damages and left heavy fire damage to the restrooms and minor smoke damage through the rest of the building, the press release states.
The Billings Fire Department responded to a fire call at the Fuddruckers restaurant in the Billings Heights early Thursday.
The restaurant is located at 875 Main Street.
The investigation into how the fire began and how much damage was caused is underway.
This is a developing story. Check back for updates.