Liam Gonzalez Bennett was just 20-months-old when he died in February 2009.
An inquest at Belfast Coroner's Court heard that the toddler had suffered 31 bruises to his head.
On Friday, after the little boy's mother Samantha Bennett wept as she denied causing the fatal injuries, it was recorded that he had died as a result of a non-accidental blunt trauma head injury.
Coroner Suzanne Anderson said it was "disturbing that no one had been made amenable" and returned the case to the Director of Public Prosecutions for review.
It is clear that Liam suffered a quite shocking assault which was so severe it was not survivable.
Coroner Suzanne Anderson
Both the toddler's mother and her ex-partner Noel McKeown had been questioned by police during a murder investigation, but no charges were ever preferred due to a lack of evidence.
Liam had been rushed to hospital after Mr McKeown, who was minding the toddler and his older sister at the time, heard him cry and then found he was not breathing when he checked on him.
The little boy was first taken to Antrim Area Hospital, but his condition was so grave that he was transferred to an intensive care unit at the Royal Victoria Hospital where he died a day later.
Concerned by his injuries, doctors contacted police and social services.
On Friday, when directly asked if she was responsible, Ms Bennett replied: "No."
The court heard that she had been out buying milk and returned to the house at Sunningdale Park in the Co Antrim town to find paramedics working on her son.
She claimed to have gone into shock at the sight.
Mr McKeown, who now lives and works in Australia, gave evidence via Skype and also denied causing the injuries to Liam.
Asked directly if he had, he replied: "No, I did not."
He could not give an explanation for the extensive bruising.
Describing how he had found the little boy struggling to breathe in his cot, Mr McKeown said he had given him mouth-to-mouth resuscitation before calling Ms Bennett and an ambulance.
He added that it was unusual for his then partner to leave the house at that time of night, around 7pm, and that there was "plenty of milk" in the fridge.
As a result of the medical evidence presented to the coroner and in view of the fact that it has been referred back to the DPP, Samantha is hopeful that the police and the Public Prosecution Service may yet be in a position to progress this matter to a final resolution.
Ernie Waterworth, solicitor for Samantha Bennett
Following the inquest, Ms Bennett's solicitor Ernie Waterworth read a statement on her behalf.
"Approximately four-and-a-half years on from the tragic and brutal death of her son Liam, Samantha Bennett is still experiencing horrendous pain with no final resolution to the case," he said.
"She is a caring, loving mother who, only three days ago, was able to finally lay her son to rest - but even that has not been finalised."
The baby's father, Juan Gonzalez Dias from Tenerife, had not been in court for the hearing.