Police are hopeful the 36-year-old woman's condition will improve, saying they aim to speak to her about the blaze that engulfed the family home west of Brisbane.
The Toowoomba house has been declared a crime scene with police treating the fire as suspicious, calling in homicide detectives.
The woman remains under police guard in a Brisbane hospital after the death toll from the devastating family house fire rose to three.
Two of her daughters, aged four and seven, died in a Brisbane hospital overnight after suffering critical burns.
A body, believed to be her nine-year-old son, was earlier found in the home's charred remains.
The death toll rose after neighbours helped six people including three children escape when the family home went up in flames in Wednesday's early hours.
Floral tributes and a teddy bear have been left outside the gutted Toowoomba home as the local community tries to comprehend the tragedy.
"I would like to extend our deepest sympathies for family, friends and all those involved in this tragic event," Detective Superintendent George Marchesini said on Thursday.
"Toowoomba is a tight-knit community and this tragedy will no doubt have a considerable impact on its residents."
The mother remains in a critical condition in hospital and police had not been able to speak with her, Det Supt Marchesini said.
However he hoped officers would be able to talk with her in a bid to understand what happened in the lead-up to the fire.
Medical advice suggested she would survive the fire, Det Supt Marchesini said, but "things can change quite quickly".
The three other people who escaped the burning home - two men aged 34 and 18 plus an 11-year-old boy - have provided statements to police.
The 34-year-old - the father of some of the children - has been released from hospital along with the 18-year-old, who is one of the mother's five kids.
The 11-year-old escaped the home unharmed.
Police will probe any history of domestic violence, if the fire was deliberate and whether accelerant was used.
"We've got scientific officers on scene today, as you can appreciate with the crime scene that was confronted, some of those investigations couldn't occur yesterday," Det Marchesini said.
"So that's ongoing through the day today as we speak."Â
The family was known to police and had some interactions in the past with officers, Det Supt Marchesini said.
"It certainly was not a premises that police were regularly attending," he said.
The Toowoomba community was on Thursday still reeling from the incident, which Queensland Premier David Crisafulli called "unfathomable and horrific".
"We send our thoughts and prayers to the people of Toowoomba and thank the neighbours and first responders who rushed to help," he said in a statement.
"A full and thorough investigation into this tragedy must take place and we'll ensure it does."
Local MP and state treasurer David Janetzki has offered support on the ground to students and staff at the kids' school, and to neighbours.
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