SMOKE detectors were installed in a house in which four siblings died but none were activated at the time, the fire service said.
Reece Smith, 19, four-year-old twins Holly and Ella and Jordan, two, died after a blaze broke out at their converted bungalow in Lytham Road, Freckleton, Lancashire, on Saturday night.
It is understood a lethal amount of smoke spread quickly from a small fire in the loft extension, with the seat of the blaze thought to be located in a wardrobe in the bedroom of the twins and Jordan.
Their mother Michelle Smith, 36, who has five other children, was led out of the house by firefighters.
The blaze took hold hours after the family held a party at the house to celebrate her birthday.
Lancashire Police said post-mortem examinations carried out by a Home Office pathologist yesterday revealed each victim died as a result of the effects of inhaling smoke. A spokesman for Lancashire Fire and Rescue Service said that smoke alarms inside had not been operative.
He said: “We can confirm there were smoke detectors in the property but they did not work.
“Forensic examination has established they were not working before the fire.
“A working smoke detector can give an early warning with quite a small amount of smoke. More notice could have been given in this situation and maybe the tragedy could have been averted.”