In 2011, Anthony O'Donohue told police he was afraid he would attack someone. Five years later, he was untreated, unsupported and set a Brisbane bus driver on fire.
An independent report into Queensland's mental health services, released on Friday, found the killing may have been avoidable.
Manmeet Sharma, 29, was burnt to death at Moorooka as he was behind the wheel of his Brisbane City Council bus in October 2016.
Mr O'Donohue threw a lit backpack containing a bottle of fuel at Mr Sharma while he was collecting passengers.
On August 10, Queensland's Mental Health Court declared he was of unsound mind at the time, and therefore not criminally responsible for his actions.
On Friday, Queensland Health published the final reports that looked at the quality of care Mr O'Donohue received between 2010 and 2016 to judge whether it influenced the attack.
A report by forensic psychiatrist Professor Paul Mullen led the investigation.
"This is a tragedy that could not have been predicted," the report said.
"Inevitably, had different decisions been made at certain times then the killing might not have occurred."
Mr Mullen reported Mr O'Donohue suffers from a delusion disorder that led him to believe he was being persecuted by the trade unions and public servants.
"Mr O’Donohue was first admitted in 2010 following a suicide attempt," he said.
"He was re-admitted a year later when he told police he was afraid he would attack those he believed were persecuting him.
"In August 2016 he was discharged with a plan for him to receive the care of a General Practitioner."
Mr Mullens said when Mr O'Donuhue was discharged he refused communication between the clinic and his GP and later tried to return to treatment at the community clinic but his attempt to gain help "failed".
"These two failures conspired to leave Mr O’Donohue in the community untreated and unsupported," he said.
Mr O'Donohue killed Mr Sharma on October 28, 2016.
Queensland Health deputy director-general John Wakefield said events like this were "incredibly rare", but did happen.
"We are a people business and nothing that people do is ever perfect," he said.
"When things go wrong ... we leave no stone unturned in understanding what happened here, what can we learn and most importantly what do we to address that."
Mr Wakefield said it was clear, from the report, that it was impossible to blame an individual for what happened.
"We will always have bad outcomes because we're a human business," he said.
"Our job is to make sure we learn from them and I'm sure from a community trust perspective, they don't expect us to be perfect, they know we're not perfect, humans are not perfect.
"What they do expect is that we will admit if we make mistakes, that we will review and understand and learn from those and we will take every action that we can to continually improve the system."
Mr Wakefield said it was not possible to determine if the incident was preventable
The reports found Queensland Health staff were compliant with legislative and policy obligations but identified several deficiencies in practices and protocols, particularly around the care provided at Metro South Hospital and Health Service and the Mental Health Service.
It was found while patients with complex mental illness were frequently assessed, there were gaps in the process.
Further, it was identified there was room for improvement in the sharing of information within the services.
A lack of integration between the forensic mental health service and community follow up was also described as problematic.
In response to the investigation, Queensland Health has new tools and staff training for violence risk assessment and the new Mental Health Act requires every health service to have an oversight committee to review risk decisions.
Further, all forensic reports are uploaded to an electronic system that can be accessed by all Queensland clinics, including GPs.
A state-wide approach to forensic mental health services will also be implemented to reduce fragmentation within the health services.
Finally, investment will be made towards capacity in the community sector to care for complex patients.
Mr O'Donohue will be held in a mental health facility for at least a decade.