Lisa Dodgson, 25, her daughters Amy Louise, aged two years and nine months, and nine-month-old Rose Marie Lakey, died in the Royal Victoria Infirmary, Newcastle upon Tyne, after the fire broke out behind the front door in their house in the West End of the city. The baby-sitter, Emma Louise Cater, 16, died last night.
It is thought all four were trapped upstairs when the fire began about 5.30am.
Detective Superintendent Derek Storey said: "We believe the fire was deliberate and an accelerant was used at the front door." Police planned to talk to members of Ms Dodgson's family and her friends to work out a picture of her lifestyle. "At this moment there is no apparent motive of why this tragic incident has occurred," he said.
"This is a tragic incident in which a young mother aged 25 and two young children died in tragic circumstances." Fifty officers were assigned to the case, he said, and a mobile police incident room would be sited in the street 24 hours a day until officers knew what had happened.
He could not confirm suggestions from neighbours that Ms Dodgson had been pregnant. Police did not know who had been the target for the attack, Mr Storey said. But he added: "I do not think for one moment that anyone would be targeting young infants. It's horrendous two young children have been robbed of their lives." Police teams were yesterday examining the house, in a terraced row in a run-down area of Denton Burn, and a sniffer dog was also called in.
The windows and doors of the property next door and many other empty houses in the street were boarded up with metal screens to keep out vandals and squatters.
Brendan O'Kane, the Tyne & Wear Fire Brigade Divisional Officer, said: "It was a very severe fire to get through, especially when persons are reported to be trapped inside.
"It is obviously really upsetting for firefighters when they have to pull children from a fire, especially a baby.
"The surviving 16-year-old girl who was in the house was in a very bad way when she went to hospital."