Flowers are laid in front of the synagogue in Copenhagen on Tuesday, Feb. 17, 2015.Michael Probst/The Associated Press
He grew up in an immigrant neighbourhood in Copenhagen and became a gang member. By his early 20s, he was a criminal with a violent temper, convicted of stabbing a man on a train. Released from prison two weeks ago, his rage turned toward different targets: a cartoonist and a synagogue.
The trajectory of Omar Abdel Hamid el-Hussein, the man who killed two people this weekend and injured five before being shot dead by police officers, epitomizes a new and disturbing challenge for law-enforcement authorities in Europe and beyond.
A 22-year-old Danish citizen, he appears to have operated without any guidance or concrete support from established Islamist extremist groups, and his reported embrace of radical ideology was relatively recent. Such violence by lone actors is difficult to predict. Preventing it requires moving beyond traditional counterterrorism techniques, like intercepting communications and infiltrating groups.
It's a task that is becoming increasingly urgent in Europe. In the choice of targets, the attacks in Copenhagen bear a distinct similarity to the deadly violence in Paris last month, when gunmen assaulted a satirical newspaper and a kosher supermarket. That suggests the shooter in Denmark drew inspiration from the earlier attacks. In Copenhagen, the cartoonist presumed to be the target of the attack was speaking at an event on free speech and survived; a filmmaker, who was attending the talk, was killed.
Not counting recent events in Copenhagen, at least six "lone wolf" attacks linked to the aims of Islamic State militants have taken place in Europe, North America and Australia since last May, including ones in Ottawa and Saint-Jean-sur-Richelieu, Que.
"You're going to see an increase in these [lone wolf] attacks," said Martin Reardon, a veteran of counterterrorism operations and former agent at the U.S. Federal Bureau of Investigation who now works at the Soufan Group.
Such actors may be inspired by existing extremist groups but they're not taking day-to-day direction from them, said Mr. Reardon. That makes their intentions hard to detect, unlike larger, cross-border plots. "When you're receiving instruction from Pakistan, Afghanistan or Syria and you're in Europe and North America, there's a good chance that's going to be intercepted," he said.
These types of attacks appear to cluster in time, added Paul Gill, a terrorism researcher at University College London. "You've got this groundswell of people who have grievances and who are motivated to act, but it's only when they see other people doing it that they realize the opportunity is there," he said. "It provides a script."
He noted that Islamic State has proved unusually successful in urging people to carry out these types of attacks, which he described as a "drip by drip, death by a thousand cuts" strategy.
It may be difficult but it's far from impossible to stop attacks by solitary actors. Mr. Gill and two other researchers recently examined 119 cases involving such perpetrators and found that in 64 per cent of cases, they had revealed their intentions to someone else – a family member, for instance, or a friend. Finding ways for people to feel safe sharing that information with the authorities is critical, he noted.
Another notable element in the emerging history of Mr. el-Hussein, the shooter in Copenhagen, is his time in prison. During his incarceration, prison authorities placed him on a list of inmates who were increasingly radicalized, information that was shared with Danish security services. Chérif Kouachi, the younger of the two brothers involved in last month's attack on the Charlie Hebdo newspaper, was also transformed by his time in a French prison. It was there that he met and befriended Amedy Coulibaly, who perpetrated the killings at the kosher supermarket.
Some experts have cautioned that attacks like the one in Copenhagen could become part of the fabric of European life for the near future. "No legislation will provide a silver bullet to this," said Frank Foley, a lecturer at King's College London, in an interview last month. He noted that there is already extensive counterterrorism legislation in place in European countries. "We have to become resilient," he said, and "to accept that this is the price of a free society."