Xavier, 3, died 16 months ago after falling ill from a suspected virus. Source: Supplied

But, yesterday police revealed that they were investigating his boy’s death as a murder.

“I just want to know what happened to my son … He was my little buddy,” he told The Courier-Mail.

RELATED: Police investigate murder of two children and attempted murder of two others — from same Morayfield family

Xavier died 16 months ago after falling ill suddenly from a suspected virus.

But police revealed yesterday that his death is now considered a possible homicide, connected to a house of horrors at Morayfield, north of Brisbane, where they are investigating the murders of two children and the attempted murders of two other children from the same family.

Mark Wildman thought he lost his son Xavier to complications from a virus. Source: Supplied

Xavier’s death was the first incident in July 2013, but Mr Wildman was not living with his son at the time.

A second child — a seven-month-old baby boy — died in the house on September 27 this year. Two other older children, aged 9 and 11, escaped from a burning bedroom in the house fire earlier this year.

Detective Acting Superintendent Damien Hansen told a press conference yesterday 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, but investigators are now of the opinion that both children were in fact murdered.

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

At the same address, he said 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.

Mr Wildman — who separated from the boy’s mother before Xavier’s death — said his son was a “really healthy little boy”.

“When I got the call that night he was at the hospital, something didn’t add up — he was a healthy three-year-old boy,” he told The Courier-Mail.

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

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 under way 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,” he said.

“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.”

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.

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

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.

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.

Read more in The Courier-Mail.