A 13-year-old boy convicted of manslaughter was supposed to be taken into CYF care a month before he killed an Auckland dairy owner last June.
But he continued to live with his mother in a well-known drug den because social workers couldn't find him.
The boy was charged with murdering Arun Kumar, the owner of the Railside Dairy in Henderson, but was convicted of manslaughter. His co-accused, just 12 at the time, was acquitted of manslaughter.
The Weekend Herald has discovered the boy, who has name suppression pending an appeal, was deemed an "at-risk child in need of care and protection" after a family group conference in May last year.
He remained in the home, addicted to synthetic cannabis, until his arrest the following month because his Child Youth and Family file was transferred between branches and social workers did not know where he was living.
His mother - who was selling synthetic cannabis in the boarding house in West Auckland where she lived with her son - also rang CYF in the three months leading up to the robbery gone wrong.
Previously she had responded to his synthetic cannabis addiction by giving him real cannabis.
A Weekend Herald review of evidence at the High Court trial, as well as interviews and other documents, found state agencies had regular contact with the dysfunctional household but failed to act decisively at several crucial moments.
Apart from CYF's inability to find the 13-year-old a month before the killing, the records show:
• When he was 4, a neighbour told CYF that his mother was using P and heroin in front of her children. CYF agreed her children needed protection but did not take them away. CYF received 20 notifications about the family, including 10 for violence, but did not remove the children.
• When he was 8, he was hit by a car and suffered a serious brain injury, but received no rehabilitation treatment from ACC. Neuropsychologist Dr Valerie McGinn told the court this was "medical neglect", which meant his brain became easily overloaded and tried to shut down in response to the pain.
• After six months, teachers found him banging his head against walls at school and saying he wanted to kill himself. His concerned principal referred him to youth mental health clinic the Kari Centre, which refused to take him and did not pick up on the brain injury.
In sentencing the now 14-year-old killer to six years in prison, Justice Graham Lang said his childhood was "turbulent in the extreme" and helped explain the jury's verdict to acquit him of murder.
Lawyer Maria Pecotic said her client was repeatedly let down by his mother and "overlooked" by the state agencies who were supposed to provide a safety net for him.
"This case is the epitome of neglect and the failure of the welfare state. And at what cost? Two lives."
A CYF spokeswoman confirmed the Office of the Chief Social Worker is conducting a review of the case. "While neither of the boys was ever in Child, Youth and Family care we need to understand if there was more that we and other agencies could have done."
The case was "tragic but not at all surprising", said Children's Commissioner Dr Russell Wills.
"The first thing I would say, is the family have failed this child. And it's not at all surprising that a boy born and raised into a household with drugs and domestic violence would have this ending.
"There won't be any one culpable agency: everybody let this boy down. All those reviews find the same thing. No one takes responsibility. Information is not shared. When families fail to engage, they aren't followed up. This is not surprising. In fact, it's heartbreakingly typical."
Minister for Social Development Anne Tolley told CYF she expected a "full review of their involvement with the boys in this terrible case" and is waiting for the report.
"If more could have been done then we need to know so that valuable lessons can be learned."
At the same time, she said an overhaul of CYF is taking place to work better for vulnerable children and information sharing has been identified as an issue.
A new Children's Team in Hamilton will also pilot the Vulnerable Kids Information System, where appropriate and secure information will be shared by relevant agencies, to try to prevent any children falling through the cracks.
Dr Wills' comments followed the release of his damning report on CYF which found children in state care are being moved up to 60 times between multiple foster carers because the agency is not giving enough attention to their long-term care.
The first of what he plans as annual reports found the agency is focused on "front-end" investigations as it struggles with 150,000 notifications of possible child abuse or neglect each year, but does not provide enough ongoing supervision and support to foster carers and staff looking after 5133 children in state care.