Yet she was assessed as not posing a risk to others and discharged from specialist mental health support and assigned a team looking after people with moderate depression. Before she was due to have her first appointment she had already killed Casey.
A serious case review published by NHS Doncaster concluded that it was as if she was "not really heard".
Summing up the treatment she received in the final months before the killing it concludes: "Everybody knew a little about [her] but nobody had the full picture."
It concluded that there had been no "systematic approach to risk assessment" looking back into her past.
The Rotherham Doncaster and South Humber NHS Foundation Trust, which was responsible for her care, said that "with hindsight" she should have been treated as a higher risk but that no single individual was responsible for the failing and no one would be disciplined.
The inquiry found that the failings followed a pattern throughout Bonser's life in which despite being seen by numerous social workers, GPs, hospital staff, and charities no single person ever had responsibility for her case and that she had been "almost invisible" in the system.
The review highlighted failings to intervene dating back to her early childhood, when she was brought up by Mormon parents who were allowed to home school her despite warnings of neglect.
When social workers visited her home at one point they found rooms "full of dead cats and excreta" but even then, the report concludes: "The issue that the children were being neglected seems to have been lost."
She later spent time in care but left at 16 and went straight into a life of homelessness, cannabis and alcohol abuse and repeated self harm.
She had a string of hospital admission late last year including after two overdoses and on January 6 she turned up at A&E at Doncaster Royal Infirmary, with her bags packed and asking to be "locked up".
She was later sent home where she continued to receive treatment from a specialist mental health team but was discharged for its care on January 30.
"There is no convincing evidence that [her] mental state had improved during January to the degree that she should have been discharged from the home treatment team," the report finds.
"There is more evidence leading to the conclusion that her mental health had deteriorated.
"The discharge plan does not demonstrate that [she] had been listened to."
It goes on: "What is evident is that she told many agencies of her concern that she was going to harm someone.
"Assessments made by them did not identify a high level of risk of this occurring.
"It is hard to establish if this was because she was not really heard."
Christine Bain, chief executive of Rotherham Doncaster and South Humber NHS Foundation Trust, said: "Our thoughts are with Casey's family and friends at this difficult time.
"We accept all the findings and recommendations made following this investigation and we acknowledge that our service to Miss Bonser should have been much better.
"We have already taken action to improve our services to patients and service users and a detailed action plan is being implemented."
Last week Doncaster Children's Services were heavily criticised in two reports into the kidnap and torture of two boys by two other boys in Edlington near Doncaster.
An Ofsted report found that child protection standards have got worse there in the last year despite Government intervention.
