Police said the pair had a power generator in the cellar that may have leaked, making inhalation of carbon monoxide the probable cause of death.
“We’re viewing it as an accident. But we can’t ascertain the cause of death with any certainty until we have carried out a post mortem examination,” police spokesman Bengt-Olof Lundberg told news agency TT.
Relatives of the pair entered the house in a small village near Sollefteå when neighbours reported not having seen the pair for more than a week.
The pair generally used the cellar door to enter and leave the house, and both of the deceased were wearing outdoor clothing when their bodies were discovered. Their dogs were also found dead at the scene.
As the couple temporarily lacked electricity, they were reliant for power on the generator they had placed in the cellar. Fumes from the generator were routed through a pipe that the couple had rigged to a fireplace, an arrangement police think may have been unsafe.
“We believe that’s how the fumes leaked into the house,” said Lundberg.
A colourless, odourless and tasteless gas, carbon monoxide is highly toxic, with studies suggesting it may be the cause of up to 50 percent of fatal poisonings in many industrial countries.