Teenage killer Craig Mulligan showed the "hallmarks of a psychopath" in his interview with police, according to a body language expert.
Mulligan was just 13 when he killed five-year-old Logan Mwangi alongside stepdad John Cole, 40, and Logan's mum Angharad Williamson, 31.
His identity came to light on Thursday after he was sentenced to life detention with a minimum of 15 years and an anonymity order banning the press from naming him was finally lifted.
Footage was also released by South Wales Police of Mulligan being questioned by detectives over CCTV showing him and Cole carrying Logan's body out of the family home.
Leading psychotherapist Nick Davies said that Mulligan's movements in the interview clip indicate he was "hiding a part of himself", reports the Daily Star.
He said: "[Mulligan] has the hallmarks of a psychopath by not showing any guilt or remorse and trying to play the innocent victim.
"But only when he says about not knowing what was in the plastic bags they threw in the river does he tuck his right hand in his sleeve, which is usually when we are hiding a part of ourselves when talking, then he pushes his right shoulder up to his ear, often associated with anxiety and self comfort."
Logan was found dead in the River Ogmore, South Wales, in July last year with injuries similar to those from a high-speed car crash.
Throughout the interview, Mulligan maintained he did not know what happened to Logan, claiming he had woken to hear the child's mum shouting 'Logan is dead'.
Mulligan told police: "I woke up in the middle of the night. Then dad come (sic) 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 and everything. I don't know what was in the bags, I can't remember.
"Then about 20 minutes after we went back out to check whether they had gone down. We didn't say anything. We were trying to be as quiet as we can because we didn't want to wake Logan or Angharad.
"I don't know whether Logan was still there at that point because we didn't look."
Police were also heard telling Mulligan that Williamson had blamed him and Cole for Logan's death, before he retorts with: "You can tell my mum to f*** off for me.
"She's blaming me and my dad for everything."
The teenage killer was once described as "pure evil" by foster carers who looked after him weeks before Logan's death.
But despite being described as a “monster” with a “desire for violence”, social services allowed him to live with Cole and Williamson.
He also had a history of "cruel" attacks on young children, vulnerable adults and small animals and was trained in martial arts.
