A man whose son shot his neighbour dead then fatally crashed his motorbike says the NHS failed to act on warnings he had severe paranoid schizophrenia.
Alex Sartain, 34, shot James Nash, 42, in the head with a homemade shotgun, outside his home in Upper Enham, near Andover, Hants, on 5 August.
And John Sartain says mental-health services had failed to act when he asked for his son to be sectioned.
Southern Health said it was "reviewing its contact" with the Sartain family.
Two hours after the shooting, as he was chased by police, Sartain crashed his 1,000cc motorbike on a country road.
Mr Nash, a well-known artist, children's author and parish councillor, died in hospital in the early hours of 8 August.
John Sartain said his son had deteriorated into a state of extreme paranoia during the weeks and months before the attack.
And he had warned the crisis team he needed urgent help.
He said: "They said, 'No, we can't do anything unless he's committed a crime or he's a threat to life.'
"Within two weeks, he has taken a life.
"I just couldn't believe it really.
"It was absolutely ridiculous.
"But that's what they said - nothing they can do.
"I just find that hard to believe that, you know, after being sectioned twice."
John, who runs a vintage-motorcycle repair business from the home he shared with Alex, said the mental-health team's failure to act on his warning had meant he had been left alone to try to manage his son as his condition worsened.
"He would just go and stand out there ranting and raving about people who had tortured him," he said.
"It was always Nasa, Boeing, the FBI - and they were all spying on him.
"Never ever in my wildest dreams would I imagine he was going to do something like he has done, ever.
"He wasn't a cold-blooded killer - it was a mental-health problem."
John also said his son had been let down by the lack of community care after he had been released from his most recent in-patient stint, last December.
And during the only recent visit a community mental-health team member had made to their home, she had not actually seen Alex.
"She didn't ask to see him or anything, just my opinion of it," John said.
"And I said, 'Well, you know he's gone back to how he was.'
"She said, 'Oh, well, that's sort of about normal,' and that was all.
"I must admit I feel very let down and angry with all of it."
The latest NHS independent investigations annual report, covering 2018-19, shows there were 111 killings by people receiving mental-health services in England, accounting for one-sixth of the total homicides.
And it uncovered a series of failings in the care of mental-health patients in the run-up to these incidents, including poor risk assessments, community care and communication with families.
A Southern Health NHS Foundation Trust spokesman said in response to the Sartain family's complaints: "We were shocked and saddened to learn about this tragic incident.
"And our thoughts are with family members at such a difficult time.
"What happened is currently the subject of investigations by the police, the Independent Police Complaints Commission (IPCC) and will be heard by a coroner at an inquest in due course.
"The trust is fully co-operating with these investigations.
"And we are reviewing our own contact with Mr Sartain and his family."
The government has pledged an extra £2.3bn a year in additional mental-health funding by 2024.
And NHS England said it intended to spend £975m a year of that transforming community mental-health care.
An official added: "Each case is a tragedy for the families involved, though these remain extremely rare events with millions of mental-health patients safely receiving expert care each year.
"Psychiatrists and care providers are committed to learning from all independent investigations, with an annual report published just a few weeks ago with key recommendations for action."
File on 4's Mental Disorder and killings that could have been prevented is on BBC Radio 4 on Tuesday, 15 September, at 20:00 and available afterwards on BBC Sounds.