Queensland police in the process of investigating the suspected murder of two children and the attempted murder of two others. Photo: Queensland Police

The re-creation of a burning bedroom played a pivotal role in the investigation into the suspected murder or attempted murder of four children in the same Brisbane home, according to police.

A grandmother has emerged as a person of interest in the homicide investigation focused on what happened in a Morayfield home in a 16-month period.

First, a three-year-old child died on July 27, 2013. It was treated as a case of Sudden Infant Death Syndrome (SIDS) but now investigators suspect murder.

Police inspect how the doorways could have come to be impeded. Photo: Queensland Police

Then, there was a fire on February 27, 2014. Two children, aged nine and 11, were sleeping in the bedroom when the fire was deliberately lit and their escape route was "intentionally impeded".

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Detectives reached those conclusions after firefighters and police re-created the fire, according to Homicide Detective Acting Superintendent Damien Hansen.

"We're able to say that there was no reason that...the children couldn't open the door in time. That door had to have been held from the outside or impeded from the outside. The children were inside the room and could not exit the room," he said.

Firefighters remove a mattress burnt in the reconstruction of the house fire. Photo: Queensland Police Service

Both children suffered smoke inhalation – along with others in the house – and one of the children was treated at Caboolture Hospital for burns to the lower limbs.

Finally, a seven-month-old baby died on September 27, 2014. Initially, the death was not treated as suspicious but investigators now suspect murder.

"I regard murder as one of the most serious offences, if not the most serious offence that can occur. To have it happen to defenceless children is just mind-boggling," Det Acting Supt Hansen said.

Morayfield is a large suburb on the northern outskirts of Brisbane in the jurisdiction of Moreton Bay Region Council.

For legal reasons, police cannot disclose the address where the deaths and fire occurred, or the name of the family

But detectives have urged anyone who knows the family or saw anything suspicious in the Morayfield area on the night of the fire to contact Crime Stoppers.

The children's grandmother – who is a "person of interest"- was present when all the incidents occurred, Superintendent Hansen said.

She had not been charged or made any admissions by Friday.

He said the two children who survived the fire were in the care of Child Safety Services.

The case only came to public light on Friday after what police described as a "comprehensive and protracted investigation" led by the Homicide Group with the help of the Child Trauma Taskforce and regional and forensic investigators.

"We are very confident we have the right line of enquiry with this investigation and will have a conclusion in the near future," he said.

"The re-creation of the burning bedroom is a key focus for our investigation and it is where we have developed strong leads and our main focus."

Anyone with information that could help with investigation is urged to contact Crime Stoppers on 1800 333 000.