The Southport attack would not have happened if the killer's parents had done their 'moral' duty and reported him to police, a damning report revealed on Monday.
Axel Rudakubana's parents knew their son was hoarding an arsenal of weapons, including machetes, for at least a year but did nothing, the chairman of the public inquiry said.
Sir Adrian Fulford said their actions, combined with catastrophic failures by police, social services, mental health teams, and other Government agencies, left the 17-year-old free to kill three young girls at a Taylor Swift-themed holiday club in the Merseyside seaside town, in July 2024.
Officials also 'unacceptably' used Rudakubana's autism as an excuse for his violent behaviour and were too focused on the risk he posed to himself, rather than the danger he posed to others, the report found.
On Monday night the parents of murdered Bebe King, six, Elsie Dot Stancombe, seven, and Alice da Silva Aguiar, nine, said they were 'devastated' to learn that their children's lives 'could and should' have been saved.
They said it had been 'deeply distressing' to read the scale of 'systemic and individual failures' and how the attack was both 'predictable and preventable'.
The report's findings, in relation to Rudakubana's parents, were 'unequivocal,' they said, in a statement issued through their lawyers.
'AR's (Rudakubana's) parents failed in their responsibility to society,' they said.
Axel Rudakubana was jailed for life and ordered to serve a minimum of 52 years, at Liverpool Crown Court in January
Bebe King, six, Elsie Dot Stancombe, seven, and Alice da Silva Aguiar, nine, were all murdered in the atrocity on July 29, 2024
'He had not left the house for two years except when armed or seeking to cause harm, yet they allowed him to leave on that day knowing he was likely carrying a weapon.'
Chris Walker, a lawyer from Liverpool-based Bond Turner, which represents the parents of Bebe, Elsie and Alice, added: 'Seeing the extent of the failures in black and white is devastating.'
Sir Adrian said Rudakubana's parents, taxi driver Alphonse Rudakubana, 50, and laboratory worker Laetitia Muzayire, 54, knew their youngest child, who had stopped going to school and was violent towards them at home, had been hoarding weapons for at least a year before the atrocity.
They had 'repeated opportunities' to warn police but instead obstructed officials who tried to intervene in their son's care, were 'too ready' to excuse his actions and failed to stand up to his behaviour, the report found.
He accused the pair, who sought asylum in Britain from Rwanda in 2002, of giving 'dishonest evidence' to the inquiry to try to justify their actions.
'If AR's parents had done what they morally ought to have done, AR would not have been at liberty to conduct the attack,' Sir Adrian added.
He said their 'total avoidance of responsibility' means 'they bear a very considerable degree of responsibility' for the attacks.
Merseyside police confirmed no charges would be brought against them on Monday night.
Sir Adrian said it was 'frankly depressing' that no public agency, including Lancashire Police, Lancashire County Council, the Government's Prevent de-radicalisation programme, and NHS mental health teams, 'stood up and accepted responsibility' for Rudakubana's case.
Rudakubana pictured in the distinctive green hoodie he wore on the day of the attack. CCTV cameras caught him outside the Hart Space dance studio, in Southport, shortly before he launched the mass stabbing
The teenager was 'passed from one public sector agency to another' in a 'merry go round of referrals, assessments, case closures and hand-offs', the chairman added.
'If appropriate procedures had been in place and if sensible steps had been taken by the agencies and AR's parents, this dreadful event would not have happened,' Sir Adrian concluded.
Rudakubana was known to agencies from October 2019, when he was 13 and made several calls to Childline and admitted taking a kitchen knife into school on ten occasions.
Police were called and he was expelled but two months later, he returned armed with a hockey stick and broke another pupil's wrist.
He was sent to a special school, which made three referrals to Prevent over concerns about what he was consuming online.
He was also repeatedly referred to mental health teams and family and well-being services.
But the Prevent referrals were dismissed because he did not have a fixed ideology and the reclusive teenager was reluctant to engage with officials, who effectively gave up trying to see him.
Rudakubana was jailed for life at Liverpool Crown Court in January last year and ordered to serve a minimum of 52 years in prison before being considered for parole.