The murder of Liam Fee may well be one of Scotland's worst child abuse cases.
For most of his short life, the two-year-old suffered terribly at the hands of his mother and her partner. Their trial heard how the couple - Rachel and Nyomi Fee - also inflicted a catalogue of abuse on two other young boys.
After giving evidence at the High Court in Livingston, Sean Catherall told the BBC he had once regarded Nyomi as his best friend.
"I didn't think in a million years she was capable of any of that stuff... especially towards a baby," he said.
Rachel Trelfa, as she was then, and Nyomi Fee moved from Ryton, about eight miles west of Newcastle-upon-Tyne, to Fife in December 2011, after Rachel left baby Liam's father.
They stayed in a Travelodge until early in 2012 when they moved to a house at Thornton near Glenrothes.
The pair would later enter into a civil partnership, with Rachel taking her partner's name.
It was following the move to Fife that the abuse of Liam appears to have begun. There, they repeatedly assaulted Liam until he could take it no longer.
He died in a bedroom of the family home on Saturday 22 March 2014.
The trial heard a recording of a panicked phone call made that evening for an ambulance. Nyomi was heard saying: "Can you please hurry up, my baby's not breathing.
"I think he's dead, he's not breathing, he's white."
But she was already blaming Liam's death on another child.
She goes on to say: "He held his mouth closed, he said he held his mouth closed and his neck because he was crying, because he was trying to hurt him."
Doctors found Liam had suffered a ruptured heart from a blow or blows to his body. They also discovered double fractures of his thigh bone and arm. In total they counted more than 30 injuries on his body.
Sean Catherall had not been in contact with Rachel and Nyomi Fee for some time. But they both stayed with him a few days after Liam's death.
His account of that is chilling.
"They weren't bothered," he said.
"They were sort of laughing, joking that they were going to get sent to jail for neglect… saying 'do you think we'll get the same cell together', stuff like that."
Meanwhile in Fife, a major police investigation was under way. Officers later found a number of objects hidden within a bed which raised their suspicions. There was a metal frame, cable ties, ropes and a chain.
Eventually police had enough evidence to charge Rachel and Nyomi Fee with Liam's murder and a series of abuses involving him and other children. It had taken five months to bring the charges.
Det Supt Gary Cunningham, from Police Scotland's east major investigations team, explained the complexities of the inquiry.
"The level of abuse and neglect that's taken place over a prolonged period of time, we wanted to exact out all the details on that," he said. "We didn't want to miss some of the additional charges that could be preferred against Rachel and Nyomi."
Rachel and Nyomi Fee's trial got under way two years after Liam's death.
For several days the jury in Court 2 at Livingston High Court watch videotaped interviews with two young boys.
They had been in the house on the night Liam died. One of the boys was the child the Fees blamed for Liam's death.
During several interviews, a specially-trained police officer and a social worker sought to assure him he was safe and not in any trouble as they tried to untangle his story.
It became clear that the boy had not strangled or suffocated Liam, he had put his hand over the toddler's mouth several days before his death but Liam had been walking and talking afterwards.
What also became clear in those interviews was the terrible abuse the two boys had also suffered.
They'd been beaten and locked in a cage, partly made from a fireguard. At times they'd been tied up all night and forced to take cold showers. There were other abuses that are too appalling to detail.
Nyomi Fee also told one of the boys she'd killed his father with a saw and that their pet boa constrictor ate little boys.
The children's evidence was crucial in this case but so too was their welfare.
Alistair Gaw, president of Social Work Scotland, said the well-being of child witnesses was paramount.
"You wouldn't be getting engaged in questioning or taking lines of questioning with a child that would actually be detrimental to that child, even if ultimately, that was at the cost of a prosecution case," he said.
So, could anything have been done to save Liam Fee? Concerns were raised about him at least three times.
His nursery alerted social services after they became worried about a change in a little boy who had seemed happy when he first came to them.
Staff found a number of injuries on him and he was losing weight.
Liam's childminder had also made her concerns known a few months earlier.
Patricia Smith, who used the same childminder, also phoned social work after meeting the Fees in the street. Liam was in his buggy. Ms Smith told the court she didn't know if he was drugged or dead.
It was around now that Rachel Fee began telling people her two-year-old son had autism.
A senior Fife social worker admitted to the court that at one point Liam "fell off their radar". A member of staff went off sick and no-one else was assigned to his case. It was not until further concerns were raised that Liam's case was reviewed.
So there are questions for Fife Council and its social work department, for the NHS, for Police Scotland and other agencies.
Douglas Dunlop is the vice chairman of Fife's Child Protection Committee which represents all the agencies involved.
It has set up a significant case review.
He said: "The circumstances of supporting families in situations such as this can be complex and there were a range of agencies involved in supporting Liam and his family and the details of that will be looked at through the Significant Case Review."
The review will be chaired by Professor Jacqueline Mok, a retired consultant who was the lead paediatrician for child protection in Edinburgh. She will look at all the records and interview the staff involved in the circumstances leading up to Liam's death.
Child killings are rare in Scotland but Liam Fee's name is now added to a tragic roll call of victims who died at the hands of those who should have protected them.