Homicide investigation into child deaths, Morayfield. Stills from video from Police of the investigation. Source: Supplied

Detective Acting Superintendent Damien Hansen told a press conference this afternoon that the deaths of a three-year-old on July 27 last year and a seven-month-old on September 27 this year were initially treated as SIDS, the Courier Mail reports.

However, police were now of the opinion that both children were in fact murdered.

Queensland police with a woman (right) believed to be the grandmother of the four children.. Source: Supplied

At the same address, two children, aged nine and 11, were injured when they were unable to open their bedroom door during a house fire on the night of February 27 and the morning of February 28 2014. The blaze was later found to have been deliberately lit.

One of the children suffered burns to their legs and both were treated for smoke inhalation along with other occupants of the house.

The incidents involving all four children took place at the same home in Morayfield, on Brisbane’s northern outskirts.

Detective Acting Superintentent Damien Hansen addresses media. Source: News Corp Australia

Detective Hansen said the investigation, codenamed Operation Mike Motif and led by the Homicide Group with the assistance of the Child Trauma Taskforce and regional and forensic officers, had been underway for some time.

“We are very confident we have the right line of inquiry with this investigation and will have a conclusion in the near future.”

“The recreation of the burning bedroom is a key focus for our investigation and it is where we have developed strong leads and our main focus.”

Queensland fire investigation officers reconstruct the deliberately lit fire in the bedroom where the children were sleeping. Source: Supplied

A turning point in the probe appears to have occurred when officers from Queensland’s Fire and Emergency Service reconstructed the fire that broke out in the bedroom where the 11-year-old and nine-year-old had been sleeping.

The children had not been able to open the door and investigators found that their escape had been “intentionally impeded”.

“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 ... impeded from the outside. The children were inside the room and could not exit the room,” Det Hansen said.

Queensland fire investigation officers at the reconstruction of the deliberately lit fire at the Morayfield home which injured two young children. Source: Supplied

Det Hansen refused to discuss a possible motive in the case.

Police said they were confident of concluding the investigation soon.

Anyone with information should contact Crime Stoppers on 1800 333 000