Police found no signs of violence on the victims, who were spending a short ski vacation at the popular ski resort, but an investigation into the circumstances of their death is under way, Celje police said in a statement.

The victims were found by the owner of the house, who is unavailable for statement.

Reports by several media suggest that the cause of the carbon monoxide poisoning was one of the two furnaces in the apartment.

About a dozen people die of carbon monoxide poisoning in Slovenia every year, mostly due to malfunctioning furnaces or insufficient ventilation.

But there are also many cases of severe poisoning that do not result in death, Miran Brvar, a doctor at the UKC Ljubljana hospital, told the press today.

Carbon monoxide is the leading cause of involuntary deadly poisoning in Slovenia and across Europe. Last week alone UKC Ljubljana treated nine such cases.