A man has been arrested over the 2015 death of a toddler who had been found unconscious with "horrific injuries" in a home south of Brisbane.
Police expected to charge the de facto partner of the boy's mother with murder on Tuesday.
The two-year-old boy had been rushed to hospital after he was found unconscious in a home at Logan about 3pm on April 8, 2015, but died shortly after, police said.
Police said the boy had been rushed from the couple's home on Ariel Street, Kingston, to the Logan Hospital and then on to the Lady Cilento Children's Hospital but could not be saved.
A 29-year-old Woodridge man, who police said was still the partner of the deceased boy's mother, was arrested on Tuesday morning at home and brought into Logan Central Police station.
Detective Inspector Mark White from Logan District Child Protection and Investigation Unit said the mother, who was also home at the time, called emergency services after finding the child in his bedroom with "horrific injuries".
"The injuries were significant and the investigation team was left with no doubt whatsoever that the death of this child was not accidental or by natural causes," he said.
Police did not expect to lay any further charges over the death.
While refusing to go into the details of the injuries or the cause of death, Inspector White said medical evidence had been critical in the investigation.
"That provided us with some real clarity as to the injuries sustained to this young child and also what actions would have been required to inflict those injuries," he said.
The death prompted the launch of the two-year-long operation North Arete, run by the Logan CPIU, with the help of the Child Trauma Unit and State Crime Command.
Detective Inspector Mark White said up to two dozen officers were working on the investigation at any one time.
"The death of any child is always tragic but this is a particularly distressing case where a young child has suffered horrific injuries.
"Our team of detectives are determined to complete a robust and exhaustive investigation and are continuing to appeal for anyone with information which could assist us to come forward."
Both Inspector White and Logan CPIU Officer-in-Charge Senior Sergeant Grant Ralston praised the efforts of officers involved, who spoke to many witnesses and consulted outside experts during a particularly challenging investigation.
"You're talking about a two-year-old victim," he said.
"Our officers are obviously trained here at the child protection unit to deal with any of these situations but it was distressing for the officers, also."