A toxicology report due out today is expected to indicate that three sisters and their aunt died following a build-up of deadly fumes in their Leixlip home.
Former neighbours of the women in Sandymount, Dublin, said they often blocked ventilation because they were concerned about draughts. Gardai who inspected the Co Kildare home where the four died confirmed that a similar practice contributed to their deaths.
Gardai believe the four died of carbon dioxide poisoning after fumes from their central heating system could not escape because the house was made "airtight" by the occupants.
According to Garda sources, the women sealed off the ventilation openings in the three-bedroom house, closed all windows and blocked all doors.
Consequently the fumes could not escape and they were poisoned. Carbon dioxide takes oxygen out of the air and consequently the women would have been asphyxiated.
Their practice of sealing off doors and windows was said to have been common at their previous home in Sandymount, where they lived up to two years ago.
Gardai involved in the investigation thought it was highly unlikely the women committed suicide, although they said a final verdict would come at a coroner's inquest.
Early indications from a post-mortem by the deputy state pathologist, Dr Marie Cassidy, indicated the presence of carbon dioxide in the women's blood.
According to people familiar with the women, they shunned contact with neighbours and often locked doors and kept windows closed, even during hot days. The heating system was on full at the time of their deaths.
Gardai yesterday carried out several tests at the home, 12 Rinawade Grove, and discovered no flaws in the heating system. A Garda spokesman emphasised that residents on the road had no reason for safety concerns about their heating system.
In terms of identifying exactly how the gas was trapped, a garda involved in the investigation said: "The tests provided nothing conclusive from our point of view and we will be holding on to the house for another couple of days."