Authorities suspect arson is to blame for a Sunday house fire in Waltham and that the fire is tied to an incident where police found a rocket-propelled grenade (RPG) at another Waltham property.

The fire started around 5 a.m. Sunday on Columbus Avenue in Waltham and fire fighters arrived at about 5:30 a.m. Three people escaped the house with no injuries, according to Mower County Sheriff Terese Amazi.

The house sustained substantial damage.

Mower County Sheriff’s Office investigators executed a search warrant related to the suspected arson case Sunday at a different Waltham property and found the RPG, which led them to contact St. Paul Bomb Squad and Air Force Ordinance Disposal Unit for assistance. The RPG was later detonated as a precaution.

Authorities suspect the RPG belonged to the arson suspect.

Amazi said authorities are looking for a witness for the arson case, but no one has been arrested. The incident is still under investigation, Amazi said.

The Mower and Dodge County Sheriff’s Offices, and the Brownsdale Fire Department responded to the fire.