Fifteen years ago, on 22 July 2011, far-right terrorist Anders Breivik killed 77 people in Norway in a double attack: a car bomb detonated in Oslo's government quarter, killing eight, followed by a shooting rampage on the island of Utøya — where the youth wing of Norway's Social Democratic Party was holding its summer camp — that claimed 69 more lives, most of them teenagers. A quarter-century on, experts and security services warn that Breivik's ideological legacy continues to radicalise a new generation of extremists worldwide.

Central to his enduring influence is the 1,500-page manifesto Breivik emailed to over 1,000 recipients just 90 minutes before the Oslo bombing, asking recipients to share it widely and posting it on a far-right online platform. In the document, he railed against what he described as the "Islamisation of the world," blamed a "left-wing elite" for enabling it, and invoked the so-called "Great Replacement" theory — the conspiratorial idea that minorities are systematically displacing white Europeans. The manifesto also contained detailed instructions on acquiring weapons and building explosives. Within days of the attack, it had been translated into dozens of languages; the first translation was into Dutch. "The manifesto was the start of a chain of manifestos," says Elanie Rodermond, a terrorism researcher at Vrije Universiteit Amsterdam. "Attackers today routinely leave behind documents that copy text from earlier manifestos."

The chain of influence is well-documented. As recently as May this year, two teenagers opened fire on a mosque in San Diego, California, killing three people. Both left manifestos listing Breivik among their inspirations. Norway's domestic security service, the PST, notes that Breivik is referenced in the majority of its far-right cases, and has observed a sharp rise in radicalised young people aged 12 to 17, many drawn into extremist networks online. For younger extremists in particular, Breivik has become something of a "saintly figure" — a status that, according to PST, has intensified over the past seven years. Experts note that on every anniversary of the attacks, new Breivik memes circulate in the far-right corners of the internet.

Meanwhile, Norway's legal system continues to grapple with the challenge of punishing a mass murderer without abandoning its own principles of human dignity and the rule of law. Breivik is currently held at Ringerike high-security prison, northwest of Oslo, in a multi-room complex that includes a fitness room, a television lounge with a games console, and a fully equipped kitchen. He has been sentenced to 21 years — the maximum under Norwegian law — with the possibility of indefinite five-year extensions if he is deemed a continued danger to society. He has repeatedly sued the Norwegian state, claiming his conditions amount to inhuman treatment; critics, by contrast, describe his accommodation as a luxury arrangement. Breivik has also used his court appearances as platforms for self-promotion, regularly appearing with extremist insignia and performing Nazi salutes.

For survivors like Astrid Hoem, who hid in a cave during the Utøya massacre, the attack fundamentally reshaped their lives and their understanding of the threat posed by far-right extremism. "For many of us, the fight against extremism became a matter of survival," she has said. Despite laws and detection methods aimed at curbing the spread of extremist content, authorities have so far been unable to fully prevent the circulation of radicalising manifestos online. "With the internet, these documents can be downloaded anywhere almost instantly, so they keep coming back," says Rodermond.