The teenager who murdered five-year-old Logan Mwangi moved into the family home five days before the killing, in a decision likened by prosecutors to throwing a lit match into a powder keg, it emerged on Thursday.

Craig Mulligan, 14, was ordered on Thursday to be detained for a minimum of 15 years while his stepfather, John Cole, and stepmother, Angharad Williamson, were jailed for a minimum of 29 and 28 years, all for Logan’s murder.

After the three were sentenced at Cardiff crown court, the judge, Mrs Justice Jefford, ruled that it was in the public interest for Mulligan, who was 13 at the time of the murder, to be named.

Cole had brought up Mulligan as his own son since the boy was nine months old, but they were separated for six months before the murder of Logan and the teenager had been in local authority care.

Foster parents who looked after Mulligan claimed they had warned a social worker he had threatened to kill Logan, but allege their fears were brushed aside. A family court judge gave Cole parental responsibility for Mulligan on 26 July last year without knowing about the threats.

On 31 July Logan’s dead body was found with the sort of injuries usually found in people who have been involved in a road accident or a fall from a height.

The judge allowed the youth involved in the murder to be publicly identified as Craig Mulligan.

Williamson, 31, Cole, 40, and Mulligan tried to escape justice by dumping the child’s body in a river in the village of Sarn, south Wales, and calling police to report they feared he had been kidnapped.

When she sentenced Mulligan, the judge told him: “I am sure you acted either as your father told you to or to mirror his actions. You idolised him and wanted to gain his approval.”

She added: “You did not see Logan as a brother. When you were in foster care you referred to Logan as ‘the five-year-old’ and on more than one occasion said you wanted to kill him, no doubt because he was with the family and you were not.

“Logan was eight years younger and so much smaller than you. The pain and suffering caused to him must have been obvious to you, but you did nothing to protect and help him.”

The judge said it was right to name Mulligan so that the public could understand the family dynamics and to make sure a safeguarding inquiry into Logan’s murder could be reported properly when it comes out in the autumn.

The judge said Williamson had been a good mother until she met Cole and began to focus on him and Mulligan, adding: “Something changed and changed tragically. Your relationship shifted and Logan became superfluous.”

In a statement read out in court, Logan’s father, Ben Mwangi, paid tribute to “the sweetest and most beautiful boy”.

He said that when he heard his son had been killed he collapsed. “I felt like every fibre of my body had died and couldn’t stop crying. I couldn’t understand how something like this had happened to my son. I’m devastated I couldn’t have been there to protect him.

“I keep experiencing recurring nightmares. Logan comes to me to tell me he’s OK and to check that I’m OK. He runs into my arms and I hold him tight, but he then slowly disappears. I wake up screaming and crying.”

Earlier this year, a jury at Cardiff crown court heard that in the months before Logan was killed he vanished from the sight of authorities, with his family using the Covid pandemic as an excuse for locking him away in the “dungeon” of his small, dark bedroom.

Mulligan was a troubled child with mental health issues. He would lash out for attention and one of the few pastimes that maintained his interest was Thai boxing.

Six months before Logan’s murder, Mulligan was taken into care after he was assaulted by his mother. Staff at a care home said he was obsessed with Cole, shaving his head to resemble him.

He was placed with a foster family and threatened to kill them and Logan. He became fascinated with death, enjoyed violent video games and tried to get other children to play a murder game that involved putting them into black bin bags.

In a police statement read out during the trial, the foster family said they had flagged the threats against Logan with Mulligan’s social worker, but she had brushed them aside. In the witness box, the social worker denied she had been told of the issues. The prosecution compared putting Mulligan into the small flat to throwing a lit match into a powder keg.

An inquiry has been launched to examine whether there were chances to save Logan after it emerged the authorities knew about some of the injuries he sustained in the months before he died.

The inquiry will also look at what was known of Cole’s past. His violent history includes a previous attack on a child, and he is said to have had an interest in the National Front. The court heard that Cole hated Logan’s similarity in looks to his birth father, who is of Kenyan heritage, suggesting racism may have played a part in his attitude towards Logan.