A <a href="https://www.thenationalnews.com/world/uk-news/" target="_blank">UK</a> teenager who took part in the murder along with his step-parents of his five-year-old stepbrother has been named as he was detained for life. Craig Mulligan, 14, was sentenced to a minimum of 15 years' detention for killing Logan Mwangi. Mulligan had previously expressed a desire to kill the five-year-old. He carried out the fatal assault on Logan with his stepfather John Cole, 40, in the flat where they lived in the Welsh town of Lower Llansantffraid, Sarn. His stepmother and the youngster's biological mother Angharad Williamson, 31, stood by and “did nothing”. Cole was told by a judge at <a href="https://www.thenationalnews.com/tags/cardiff/" target="_blank">Cardiff</a> Crown Court that he would spend at least 29 years behind bars while Williamson would serve a minimum of 28 years' custody. A social services investigation is now under way into the circumstances of Logan's death as Mulligan had only returned to the care of Cole five days before the murder. The trio were convicted in April of killing the little boy in Sarn, Bridgend, South Wales. Passing sentence, Mrs Justice Jefford said: “You are responsible for Logan's death and all the anguish that has followed from it. “Because he was killed in his own home and by his own family, it is not possible to be sure exactly what happened to him but what is very clear is that shortly before his death, this little boy — three feet and five inches in height and weighing only three stone and one pound — was subjected to a brutal attack.” The judge described the injuries Logan had suffered and said they were the “sort of injuries seen in abused children”. “The inflicting of these injuries on a small, defenceless five-year-old is nothing short of horrifying.” The judge said Williamson had tried to shift the blame on to others. “You had the opportunity to protect your son from further injury and you did nothing,” the judge told Williamson. “What happened to Logan must have involved appalling mental and physical suffering. “It is impossible to imagine the terror and anguish of a five-year-old child as these horrific injuries were inflicted on him by those he regarded as family and with the complicity of his mother. “In a final act of callousness, all three of you agreed to try to cover up Logan's death as an accident. “It is impossible not to draw the inference that you regarded disposing of Logan's body as akin to disposing of rubbish.” Both Williamson and Mulligan were convicted of a further charge of perverting the course of justice, an offence Cole had admitted before trial. Logan, a previously “smiling, cheerful little boy”, was discovered in the River Ogmore in Pandy Park on the morning of July 31, 2021. Police found him partially submerged, wearing dinosaur pyjama bottoms and a Spider-Man top only 250 metres from his home. The youngster had suffered 56 external cuts and bruises, and “catastrophic” internal injuries, which were likened to a high-speed road accident. In the months and weeks leading up to his death, Logan had been “dehumanised” by his family, prosecutors said. Logan's stammer is said to have worsened, becoming particularly bad around Cole. He wet himself more frequently and began harming himself. Friends of the couple said Cole told them he did not like Logan, and others said his attitude changed after becoming obsessed with the idea that Williamson had cheated with Logan's father Ben Mwangi. After Williamson gave birth to his own child, Cole was reluctant to let Logan see the baby and later claimed the boy had tried to smother the infant. Medics made a safeguarding referral to the police after Logan suffered a broken arm in August 2020. By March, due to concerns over Cole's behaviour, Logan and his younger sibling had been assigned their own social worker. In June, a month before Logan died, the family were removed from the child protection register, meaning it was believed there was no longer a risk of significant harm. A foster family whom Mulligan stayed with claimed to have heard him say he wanted to kill Logan. On July 20, Logan tested positive for Covid-19 and he was shut in his bedroom with a baby gate barring him from leaving. Caroline Rees QC, prosecuting, said: “He had been kept like a prisoner in his small bedroom in the flat you saw, a room likened by Williamson as a dungeon.” A child practice review into Logan's death has been launched by the Cwm Taf Morgannwg Safeguarding Board.