A build-up of poisonous gases in the home of three sisters and their aunt in Leixlip, Co Kildare, is believed to have accidentally caused their deaths, according to Garda investigators.
The investigation team is ruling out the possibility of suicide or murder, but said the final verdict on the deaths would come at a coroner's inquest.
The four women, who lived at 12 Rinawade Grove, blocked the ventilation in their home and this meant the gas from their heating system could not escape, Garda sources said.
Their decomposed remains were found by the owner of the house on Wednesday.
Preliminary findings from the deputy State pathologist, Dr Marie Cassidy, on the four bodies revealed traces of carbon dioxide in their blood stream.
The women blocked up doors and windows at their former home in Sandymount, Dublin, according to neighbours, who said they were anxious to prevent draughts. Because the heat was on full power, gases would have caused asphyxiation.
Gardai carried out tests at the house yesterday and attempted to re-create the conditions in which the women died. However, these were inconclusive and further tests will be conducted.
Two of the women were officially named as Josephine (46) and Catherine Mulrooney (51). Dental records were being examined to positively identify the other two women, although they were believed to be Catherine's twin sister Ruth (51) and Frances, their 83-year-old aunt.
The women were described as "reclusive" by neighbours in Leixlip - who rarely saw them - a view supported by people living in Sandymount.