The deaths of a husband and wife who were killed by their schizophrenic son were "predictable but not preventable", an independent review has found.
Timothy Brown, 47, was receiving help from Mersey Care when he stabbed his parents to death in 2014.
Brown, of Devonshire Road, Toxteth, claimed a "spaceman" told him to kill his parents Paul, 73, and Dorothy, 66.
The review of his care and treatment found "fundamental errors" in his "structured risk management".
Niche Patient Safety, who carried out the investigation for NHS England, also found Brown had previously stabbed his grandmother's dog to death.
"It is our view that the homicide of S's parents was certainly predictable but was not preventable," the review said.
The Scott Clinic in Rainhill, Merseyside, sent a team to Brown the day before he killed his parents, but he was deemed "low risk" by a psychiatrist who felt he had not displayed "behaviour that was overly concerning".
Mr and Mrs Brown were found dead at their home the next day.
The review said a "more assertive model of care may have alerted services earlier to a change in S's condition which may have led to an earlier response" from the Forensic Integrated Resource Team which was responsible for his care.
It also "found fundamental errors in S's structured risk management" and "a general failure to prioritise risk attributable to untreated or partially treated schizophrenia".
The report revealed that Brown assaulted his parents in 1995, stabbed his grandmother's dog to death in 1996 and admitted himself into care in 1999 because he felt was dangerous and was afraid he would attack his parents.
Dr David Fearnley, medical director for Mersey Care NHS Foundation Trust, said the report revealed a complex situation but clearly found the deaths could not have been prevented.
"We know that he had been violent towards his parents before, on more than one occasion, and had delusions that involved their harm, involved killing them," he added.
But he said measures were in place "for years", even up to the day before "the very sad and very tragic events" when the team in charge of Brown's care identified that he was becoming unwell and were putting plans in place about admitting him to hospital.
"What we hope is that while this wasn't preventable, learning from this may prevent similar tragedies in the future," Dr Fearnley added.
The report noted "considerable changes" in internal quality governance in the trust since the incident and said there was "a culture of continuous improvement" there.
There were 12 recommendations including:
- Care plans for patients with schizophrenia at risk of harming family members should learn from the evidence on parricide
- Ongoing contact with family members or partners must form part of the core risk assessment
- Risk assessment training for doctors who section forensic patients and other mental health professionals