The parents of a mentally ill ex-soldier warned a health trust he was collecting knives before he murdered an 83-year-old dog walker.
Alexander Palmer, 24, stabbed Peter Wrighton 45 times in woodland near East Harling, Norfolk, in August.
A letter from a Norfolk and Suffolk NHS Foundation Trust psychiatrist said Palmer was told to “get rid” of the weapons and would “hopefully” comply.
The trust said an internal review into the case was under way.
Palmer was described as “dangerous” when he was sentenced to life in prison for murdering Mr Wrighton, whose injuries were so severe police initially thought he had been killed by an animal.
Palmer’s parents said their warnings that their son was not taking his medication were ignored because health workers said they could only “believe the patient”.
His mother and stepfather – who do not want to be named – told BBC Look East he had been documented as saying it was “inevitable” he would kill.
The parents said they also told mental health professionals Palmer had knives and added the “severity” of his illness was not appreciated.
“Alex was purchasing knives. Once he knew I knew that he’d got them, he became more secretive,” his mother said.
In January 2017, Palmer, who lived in Cringleford near Norwich, told his GP he was experiencing more voices telling him to hurt people and had bought a hunting knife and a machete.
Palmer subsequently visited a specialist at the Norfolk and Suffolk NHS Foundation Trust, who wrote to the GP.
“It is in his best interests to get rid of such weapons. Alex is aware of this and understands this and hopefully will act accordingly,” the specialist wrote.
The letter goes on to say that Palmer did not feel driven to act on his violent fantasies.
Palmer had served in an Army commando regiment but was discharged in November 2015 after his head was “crushed” in a violent attack, his parents said.
They said he returned home a “different” person but received multiple diagnoses from a string of professionals.
His mother added: “It says in the letter ‘mum was concerned about what Alex was going to do with those knives’. There was no ‘this is a mental health patient with a machete’.”
His stepfather said: “They were talking to him like he was a teenager as opposed to a trained soldier.”
His mother said it had been noted in two reports that her son “said he is going to kill somebody, how he is going to kill somebody and he’s said it’s inevitable”.
“They thought we were busybodies, interfering in Alex’s mental health and care, because they were the experts,” she added.
Correspondence sent by mental health professionals over a two-year period before the murder documented Palmer’s violent fantasies and the voices telling him to harm others.
After he was sectioned in 2015 in Peterborough, medical notes said he wanted to “get rid” of people in the street by “stringing them up and cutting them open”.
A psychologist who treated Palmer at RAF Marham contacted police after reading press reports about Mr Wrighton’s murder, saying she thought he could be responsible.
His mother said Norfolk and Suffolk NHS Foundation Trust had “missed chances” to prevent the killing.
“I am not going to blame them solely, maybe we should have done more, but the amount of times we were treated like we were interfering, it didn’t matter how desperate I was,” she said.
Palmer’s stepfather added: “Peter Wrighton paid the price and his family continues to do so.”
In a statement, Norfolk and Suffolk NHS Foundation Trust said its independently-chaired internal review would was due to finish this summer.
It said it would “incorporate information from all involved, including the families, carers and, of course, clinicians, along with any external parties who may have any relevant information”.
“The trust wishes to offer its continued support to the victim’s family and to others who have been affected by these tragic violent events,” it added.
“It would be inappropriate to pre-judge the findings of any review or to comment any further at this point.”