Southport killer Axel Rudakubana admitted carrying a knife more than 10 times but was still able to buy a blade on Amazon, the home secretary has said.

Yvette Cooper said public bodies “completely failed to identify the terrible danger that he posed” as his obsession with extreme violence developed in the years before the attack.

On Monday, Rudakubana pleaded guilty to killing Bebe King, six, Elsie Dot Stancombe, seven, and Alice da Silva Aguiar, nine, last July. He will be sentenced on Thursday.

Earlier, the prime minister warned of a “new and dangerous threat” from violence-obsessed individuals, which followed the announcement of a public inquiry into missed opportunities to stop Rudakubana.

Cooper told the Commons it was a “total disgrace” that he was “easily able to order a knife on Amazon” at the age of 17 despite having a prior conviction for a violent offence against another child at school.

She also said there would be a “thorough review” of Prevent, after it emerged that Rudakubana was referred to the anti-extremism programme on three separate occasions between 2019 and 2021.

An initial review over the summer found Prevent failed to flag Rudakubana as a serious threat because he did not exhibit a commitment to a single radical ideology, Cooper told the Commons.

“Too much weight was placed on the absence of ideology,” she said, in light of the fact he was “obsessed with massacre or extreme violence”.

Cooper said it was “unbearable to think that something more could and should have been done” to stop him, and that “action against him was much too weak”.

The home secretary said a public inquiry would be given all the powers it needed to assess whether red flags were missed. Areas of interest are likely to include:

• The five occasions Lancashire Constabulary officers responded to calls from Rudakubana’s home address between October 2019 and May 2022, relating to concerns about his behaviour
• Repeated referrals to safeguarding services, children’s social care and adolescent mental health services
• A referral to the youth offending team after Rudakubana’s conviction for a violent offence
• Concerns passed on to the local authority by Childline following calls by Rudakubana as a teen, including one where he disclosed he planned to take a knife to school because of racial bullying
• His exclusion and non-attendance at school

“There are grave questions about how this network of agencies failed to identify and act on the risks,” Cooper said.

Speaking in Downing Street earlier on Tuesday, Sir Keir Starmer said failings by public bodies in the lead-up to the Southport murders “leap off the page”.

He said that it was “clearly wrong” Rudakubana was deemed not to meet the threshold for intervention from the Prevent programme.

The circumstances around Rudakubana’s offending have led to wider scrutiny over what the government has described as an increasing threat from young people with an interest in extreme violence.

Cooper told the Commons that 162 people were referred to Prevent last year over concerns related to school massacres, amid what she described as a “wider challenge of rising youth violence and extremism”.

The number of children investigated for involvement with terrorism has increased threefold in three years, she added.

The government said tech companies must remove the type of extreme material Rudakubana had accessed online.

They “should not be profiting” from hosting content that “puts children’s lives at risk”, Cooper said.

Opposition parties have put pressure on the government over what information was released to the public about offences Rudakubana was being investigated over prior to his guilty pleas.

On Tuesday, shadow home secretary Chris Philp said “we need to know who in government knew what and when”.

But Cooper said there was a risk Rudakubana could have “walked away a free man” if the government had released information that prejudiced his ability to have a fair trial under UK law.

However, the home secretary acknowledged the spread of misinformation on social media “puts those long established rules under strain” and said contempt of court laws were being reviewed by the Law Commission.

Misinformation about the Rudakubana’s identity spread in the aftermath of the Southport attack, fuelling violent disorder.

Posts falsely claiming Rudakubana – who was born in Cardiff – was an asylum seeker who had arrived in the UK on a small boat were shared widely online.

More than 1,000 people have been arrested over the subsequent riots and hundreds have been charged and jailed.