Britain is facing a new and dangerous form of extremism, Prime Minister Keir Starmer said on Tuesday, warning that loners and misfits were being radicalized by a “tidal wave of violence freely available online”.

ua speech in Downing StreetMr. Starmer said that unlike the terrorist threat posed by organized groups like Al Qaeda, where a clear ideology can be identified, some young people become fixated on extreme violence for its own sake. He compared the brutal killing of three girls at a dance class last July in Southport, England, to school shootings in America.

On Monday, Axel Rudakubana, 18, confessed to the murder of three girls in the coastal town of Southport and the attempted murder of 10 more people. Attack last summer caused riots in several towns and cities across England and Northern Ireland.

Serena Kennedy, chief constable of Merseyside Police, the force that covers Southport, said in the statement on Monday that Mr. Rudakubana had “an an unhealthy obsession with extreme violence”, as evidenced by the multitude of documents, images, videos and texts about violence, conflicts and genocide that he watched on his digital devices.

“We know that he has researched numerous documents on the Internet that show this obsession,” she said and added: “From all those documents, no ideology has been exposed and that is why this was not treated as terrorism.”

The government announced a public inquiry on Monday after it emerged the perpetrator had been referred three times to a counter-terrorist program called Prevent when he was 13 and 14 because of his interest in extreme violence. Because it was not considered to be motivated by terrorist ideology, it was not considered suitable for intervention.

Mr Starmer said the judgment was “plainly wrong” and he would not allow any government institution to “back away from its failure”, which, “in this case, frankly, jumps off the page.”

He rejected claims by the British right-wing media that there was a cover-up in the killings, saying he was informed of the police investigation while it was ongoing, but was barred by law from revealing information about the perpetrator before any possible trial. Strict rules govern the disclosure of information during active court proceedings in Britain to guarantee the right to a fair trial.

“If this trial had failed because I or anyone else had discovered key details while the police were investigating while the case was being built, while we were waiting for the verdict, then the vile individual who committed these crimes would walk away as a free man,” said Mr. Starmer.

In the days after the July 29 attack, right-wing critics suggested that information about the perpetrator was withheld to contain public outrage. The unrest erupted after the rapid spread of misinformation about the killer’s identity — including false claims that he was a recently arrived undocumented immigrant from Syria. Mr. Rudakubana was born in Wales.

Police announced in October that after searching Mr. Rudakubana’s home, they found ricin, a deadly poison, and a PDF file titled “Military Studies in Jihad Against Tyrants: An Al Qaeda Training Manual.” But in statements Monday, investigators made clear that the killer’s interest in violence was broad and did not appear to stem from any single ideology.

The attack in Southport was “a sign”, said Mr. Starmer, that terrorism is evolving and that Britain is facing a new threat with more organized groups such as those linked to or inspired by Al Qaeda.

“We also see acts of extreme violence committed by loners, misfits, young men in their bedrooms, accessing every possible material on the internet, desperate for notoriety,” said Mr. Starmer, adding that such people were sometimes inspired by traditional terrorist groups. , were “fixated on that extreme violence, seemingly for its own sake”.

Since such acts are designed to terrorize, said Mr. Starmer, Britain’s anti-terror laws may need to change to recognize and deal with the new threat.

“I think it’s new — you’ve seen versions of it in America with some of the mass shootings in schools,” he said. “It’s not an isolated, gruesome example. In my view, that is an example of a different kind of threat.”

Mr Starmer claimed the tragedy of the Southport murders “must be a line in the sand for Britain.” He said he would deal with issues that are “far-reaching, unencumbered by cultural or institutional sensitivities and driven only by the pursuit of justice.”

Chris Philp, who speaks on behalf of the opposition Conservative Party on Home Affairs, welcomed the establishment public inquiry but said it needed to investigate “what the government knew when”, whether authorities were “as open and transparent with the public as they could be” and whether a lack of transparency contributed to the riots.