Investigators' growing certainty that a single gunman was responsible for the Christchurch mosque attacks that claimed 50 lives has renewed attention to a longtime concern: terror attacks by ideologically driven lone actors in the US and Europe.

The shootings in Christchurch, New Zealand was "a blatant imitation. This is a copycat crime. He's followed others who have come before him," said Mark Hamm, a professor of criminology at Indiana State University who has charted such attacks in the US.

But the public's stereotypes of such "lone wolves" risk obscuring the fact that many are not nearly as solitary as they might seem, criminologists say.

"They may be alone at the time of the attack," said Noemie Bouhana, a professor of security and crime science at University College London who studies terrorism. "But the ties have existed that have been necessary for the attack to occur, and I would be very surprised if that wasn't the case here."

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Flowers lay at a memorial near the Masjid Al Noor mosque for victims of the Christchurch attacks. Photo / AP
Flowers lay at a memorial near the Masjid Al Noor mosque for victims of the Christchurch attacks. Photo / AP

Those ties are key not just to prosecuting such terrorist attacks but to finding ways to prevent them, experts say. That helps explain investigators' determination to follow all threads, even with the 28-year-old Australian they say was the lone gunman already in custody.

"We believe absolutely there was only one attacker responsible for this," Mike Bush, New Zealand's police commissioner, said at a news conference. "That doesn't mean there weren't possibly other people in support and that continues to form a very, very important part of our investigation."

Those looking to unravel how the attack took shape have decades of history to consult. Attacks by lone actors harbouring extremist motives date back decades, particularly in the US.

In the 1940s and 1950s, electrician George Metesky planted more than 30 bombs in New York theatres, libraries and other public places. He was driven by anger at his former employer over a workplace injury.

Theodore Kaczynski, the technophobe known as the Unabomber, was arrested in 1996 after nearly two decades of planting bombs that killed three; Eric Rudolph spent years as a fugitive while carrying out a series of anti-abortion attacks, including bombing a park during the Atlanta Olympics. In 2015, Dylann Roof slaughtered nine at a historically black church in Charleston, South Carolina.

The vast spaces and lack of borders in the US, along with a culture of individualism, help seed such attacks, Hamm said.

A photo tribute for Christchurch mosque shooting victim Tariq Omar lies amid mounds of flowers across the road from the Al Noor mosque. Photo / AP
A photo tribute for Christchurch mosque shooting victim Tariq Omar lies amid mounds of flowers across the road from the Al Noor mosque. Photo / AP

But access to more powerful guns and ammunition has increased the lethality, said Hamm, who with a fellow researcher used a Justice department grant to identify more than 120 lone wolf attackers in the US over seven decades.

Hamm points to the 2009 shootings at the Army's Fort Hood in Texas that left 13 dead. Maj. Nidal Hasan, convinced that US wars in Afghanistan and Iraq were an assault on Islam, used a pistol equipped with a laser sight and magazine extenders to fire more rounds.

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But terrorist attacks by loners have also increased around the globe. Between 1990 and 2017, the US saw 56 attacks by ideologically driven lone actors, a study by Bouhana and others found. Over the same period, Europe and other countries were targeted by 69 such attacks, they concluded.

The Christchurch attack has drawn comparisons to the 2011 massacre of 77 people in Norway, most at an island youth camp. The attacker, Anders Behring Breivik, raged against Europe's growing Muslim population and claimed to represent what turned out to be an imagined order of Knight crusaders.

In the years since the September 11 attacks in the US, law enforcement agencies in many countries have stepped up efforts to detect and prevent plots by large groups.

Muslim reactionaries in Europe have continued to carry out attacks organised in small cells. But they have increasingly followed right-wing counterparts in acting alone, said Tore Bjorgo, director of the Centre for Research on Extremism at the University of Oslo in Norway.

People mourn at a makeshift memorial site near the Al Noor mosque. Photo / AP
People mourn at a makeshift memorial site near the Al Noor mosque. Photo / AP

"The security environment makes it very difficult to operate as an organisation, as part of a large group ... so the only viable option is to go as a lone actor," he said.

That does not mean lone attackers are entirely disconnected from others. Even Breivik, who planned his attack in isolation, had been involved in a right-wing political party and wrote frequently on political websites, said Bjorgo, an expert on the underpinnings of the Norway massacre.

Rather than plotting entirely in their own heads, lone extremists increasingly find their inspiration in what others post online, experts say. And many are prone to discuss their motives or their plans, offering one of the best chances for preventing attacks.

"They tell people why they want to engage in violence. They tell people what their grievances are. They even tell people, in some cases, what they're going to do. It's just in a lot of cases, for whatever reason, people don't report it," Bouhana said.

She and other European experts take issue with the "lone wolf" description often used in the US, saying it promotes a false mystique about such attackers as isolated from others and exceedingly stealthy.

But looking beyond that mythology, it is clear that such attacks are characterised by troubling commonalities in the mindsets of those who carry them out, criminologists say. While there is still much we don't know about the prime suspect in the New Zealand attack or how it was planned, early details provide red flags.

- AP