Seven people have died of carbon monoxide poisoning in their homes in the United States - including several children and teenagers.
Relatives of the family had already discovered the victims over the weekend in Moorhead, Minnesota.
Forensic medicine announced the cause of death on Wednesday.
In addition to three adults, the dead included two children aged 7 and 9 and two young people aged 16 and 19.
According to the police, there are no indications of a crime.
According to investigators, the carbon monoxide came either from the house's stove or from a car in the garage.
A malfunction of the stove could not be determined, said the police.
The blood of the victims will now be examined in further tests to determine whether the poisoning can be attributed to exhaust gases from a vehicle.
Investigators also discovered a carbon monoxide detector in a closet in the laundry room, the batteries of which had been removed.
The other sensors in the house were devices that can only detect smoke.