A teenager who was one of three people who murdered Logan Mwangi turned on the boy's mother in a police interview after his arrest, footage has shown.
John Cole, 40, Angharad Williamson, 31, and 14-year-old Craig Mulligan were given life sentences for murdering five-year-old Logan .
His body was found in the River Ogmore, near his home in Sarn, Bridgend county, in July 2021.
Some police officers said it was "one of the worst cases they had worked on".
Logan was murdered by his mother Williamson, stepfather Cole, and Mulligan, who was raised by Cole since he was nine months old.
He died after suffering a "brutal and sustained" attack at home, which left him with 56 "catastrophic" injuries, before his body was dumped like "fly-tipped rubbish".
Police footage of Mulligan has come from ITV Hub's The Murder of Logan Mwangi.
It shows the teenager - who had moved in with Cole and Williamson just four days before Logan was killed - being interviewed by South Wales Police officers.
It showed Mulligan, who was 13 at the time of his arrest, swearing and lying to officers.
When questioned about the CCTV that showed him and Cole walking in the middle of the night to dump Logan's body in the River Ogmore, Mulligan said: "I woke up in the middle of the night, then dad [Cole] came into the living room with me.
"I know we weren't meant to do it, but we grabbed some of the rubbish in the back garden and we chucked it in the river - it was all in black bags."
Mulligan told police he didn't know what was in the bags, but that 20 minutes later, he and Cole went back to check whether they had "gone down".
He added: "We were trying to be as quiet as we could, because we didn't want to wake Logan or Angharad, I don't know whether Logan was there, because we didn't look."
Mulligan then swore repeatedly at officers when referring to Williamson blaming him and Cole for everything, stating they had not done anything and she was saying anything she could to be released.
At Cardiff Crown Court, the judge, Mrs Justice Jefford, said Mulligan did not see Logan as a brother and when he was in foster care, he referred to Logan as "the 5-year-old" and on more than one occasion said he wanted to kill him.
She said there was no doubt this was because Logan was with the family and Mulligan was not.
'Hit him three or four times'
Police interview footage also showed the moment Williamson cracked, when asked about the pyjama top officers found, and the damage to it appearing consistent with the struggle and Logan's injuries.
Williamson said: "I don't know about his injuries, if I did I would have taken him to the hospital."
After officers told her they wanted to find the truth, Williamson then said Cole had hit Logan.
"Jay [Cole] hit him three or four times in the stomach because he wouldn't talk, he said the only way this boy understands is pain. I was like 'no, it isn't just stop'.
"He was laughing, saying 'that's a good one isn't it?'.
"Craig hit Logan as well, he swept him, and pushed him to the floor, and he banged his head, Jay [Cole] said he had to do it to get him to listen," she said.
'Most traumatic scenes'
The footage has been made available as police officers spoke of the trauma they experienced while investigating Logan's murder.
Response officers had been called to the area by Williamson, who claimed Logan had gone missing. But the call was later revealed to be part of a plot by Williamson, Cole and Mulligan to cover up his killing.
The trial earlier this year saw members of South Wales Police give evidence, with some breaking down in tears while recalling the distressing scenes they witnessed.
Those who found Logan, including PC Lauren Keen who carried his body out of the river, and paramedics who treated him at the scene were said to be deeply disturbed by what was described by Det Insp Lianne Rees as "the most traumatic scenes imaginable".
Investigator and family liaison officer Det Con Clare Edwards recalled how she had gone to the Princess of Wales Hospital in Bridgend where Logan had been taken to get a first account from Williamson.
She said she took her in to see Logan in a room in the hospital and began to cry as she described seeing a bruise above his left eye.
Det Sgt Ed Griffith said it was one of the worst cases he had worked on.
He said: "10 out of 10 on the scale, to be honest, this will live with me forever."
Iwan Jenkins, deputy chief crime prosecutor for CPS Cymru Wales, said the situation was very traumatic.
"When you see a cold, calculated murder of this nature, it does impact on people, there's no doubt about that," he said.
"This was quite a horrific case with the injuries caused, the planning to dispose the body, the lying to the police previously.
"And while we are pleased that we've been able to bring some form of justice to the situation, the matter remains that we cannot bring Logan back."
Cole was sentenced to a minimum term of 29 years in prison, Williamson will serve at least 28 years, and Mulligan will be detained for at least 15 years.