Watertown firefighters responded Sunday at 7:23 a.m. to 1118 River Drive for a reported explosion and subsequent fire.
Watertown Fire Chief Greg Michalek said when firefighters arrived on the scene the back of the house had been blown out.
He said firefighters could not enter the building initially due to structural concerns, and the fire caused the roof to collapse. He said the house will need to be torn down.
Michalek said there was nobody in the house at the time of the explosion.
Jesse Fremont is the property owner. Watertown police have not released the names of the tenants.
Five fire departments from surrounding areas and an ambulance from Ryan Brothers Ambulance Services of Fort Atkinson were at the scene assisting. Ambulances from Lake Mills and Juneau came to the Watertown fire station to cover the city.
Crews were on the scene until around noon Sunday, and a section of River Drive, from Dakota to Hyland streets, was closed for a time.
Michalek also said there was one injury when a firefighter twisted a knee during the operation, but it was not serious.
He said the state fire marshal's office, the Division of Criminal Investigation and Watertown police and fire departments are investigating the cause of the fire. An engine from the Watertown Fire Department stayed at the scene assisting with the investigation until around 7:30 p.m. Sunday. The investigation is still ongoing.
The First Congregational United Church of Christ, 120 Kuckkan Lane, is collecting financial donations for the family, a mother with two children who lost all of their Christmas gifts, a girl and a boy. Donations for the family should be marked either for the house fire or "Love Fund." Donations will go toward providing food, clothing and, if necessary, providing a place to stay.