Axel Rudakubana, then 17, launched a frenzied knife attack in northern England's Southport on July 29, 2024, killing Bebe King, six, Elsie Dot Stancombe, seven, and Alice Dasilva Aguiar, nine. Rudakubana also wounded 10 others in the attack, which was followed by days of nationwide rioting.
He was jailed for at least 52 years after he admitted the killings shortly before his trial in 2025.
Inquiry chair Adrian Fulford, who described the killings as "one of the darkest moments in recent national memory", said there were five key failings before the attack.
These included a failure by agencies - including the police, the counter-radicalisation scheme Prevent and social services - to "take ownership of the risk" Rudakubana posed despite many warning signs, Fulford said on Monday.
Rudakubana's parents "bear significant responsibility" for failing to alert the authorities to the risk their son posed, he said.
Fulford also cited poor information sharing between agencies, Rudakubana's autism being used "as an explanation, or even an excuse, for his conduct" and the lack of any oversight of his online activities.
Rudakubana was referred to Prevent three times, first in December 2019 after taking a knife to school and searching online for school shootings.
He also attended his previous school that month with a hockey stick and a knife, which Fulford described as "a watershed event".
"If appropriate arrangements and reasonable resources had been in place to address the risk that (Rudakubana) posed to others from December 2019 onwards, it is highly likely that the tragedy ... would not have occurred," Fulford said.
He called for a significant change in how society handles "violence-fixated individuals".
The inquiry's second phase will examine the wider issue of children being drawn into violence, an increasing concern for British authorities.
The second phase would also consider whether there should be additional powers on top of those in Britain's Online Safety Act, to restrict or monitor internet access for young people who show signs of being fascinated with violence, Fulford said.
When he ordered the inquiry, Prime Minister Keir Starmer said its findings could lead to a change in terrorism laws.
Starmer said Monday's report was "profoundly disturbing", adding, "I'm determined to make the fundamental changes needed to keep the public safe."
