LONDON Often radicalized unseen and online, "lone wolf" attackers -- as authorities suspect the gunman who killed 76 people in Norway will turn out to be -- are tough to detect and may pose a growing challenge to authorities.
Anders Behring Breivik claims to be part of a wider network of so-called "Knights Templar" crusaders with additional cells across Europe.
Many security experts doubt such a network exists and if it does its members may be operating almost entirely independently -- and therein lies the greatest threat.
In the decade since the September 11, 2001 attacks, authorities have concentrated on building up surveillance systems that have made it easier to shut down conventional "terrorist networks" by tracking interpersonal connections, methods largely useless against someone working alone.
The good news has been that "lone wolves" are often inept and lacking in the expertise to pull off big attacks, security experts said. Now Breivik appears to have raised the bar.
"If he was genuinely operating solo, it's disturbing to security officials," said John Bassett, a former senior official at British signals intelligence agency GCHQ and now associate fellow at the Royal United Services Institute.
"There have been several examples of radical individuals acting on their own but this man would be distinguished by his imagination --- from his point of view -- (and) effectiveness."
More and more frequently these days, experts say, online extremist activity is less about militant groups deliberately recruiting people to their cause and more about vulnerable and isolated individuals looking to find a sense of community they lack elsewhere.
"You can radicalize yourself online," said Richard English, director of the Center for the Study of Terrorism and Political Violence at Scotland's University of St Andrews. "It's much harder to the authorities to keep track of."
NOT ENTIRELY UNDETECTABLE
Hard, but not entirely impossible. Scott Stewart, a former U.S. law enforcement official with experience tracking right-wing militants and now working for private intelligence company Stratfor, says there are key times at which even the most secretive "lone wolf" is open to detection.
"There are still points where they are vulnerable," he said. "When they carry out close reconnaissance on their target, they tend to stand out and look like they are behaving strangely. You should still be able to pick them up when they buy explosives for ammunition."
Breivik's online journal makes it clear he realized the moments he was most exposed were when he was looking to buy fertilizer for explosives or weaponry.
His purchase of an automatic rifle for "hunting" seems to have gone unnoticed, and although the fertilizer purges was flagged up by authorities as potentially suspicious the matter was apparently not pursued.
Officials say they are beginning to take a much broader approach by trying to detect those who might conduct attacks much earlier -- but critics say the handful of initiatives so far are too narrow and fail to address the underlying issues.
Speaking to Reuters earlier this year, Norman Bettison -- the lead official at Britain's Association of Chief Police Officers against violent extremism -- said several school-aged children in Britain had been referred for specialist anti-radicalization education after developing what he called "unsavory right-wing extremist views".
In Holland, police have begun monitoring social networking and other websites to try to detect dangerous individuals and illegal hate speech in advance. But many question how successful -- or even desirable -- such approaches can be.
"The challenge... is to produce good intelligence that gives advanced warning of attacks without either suffocating in huge masses of irrelevant material or intruding unduly into civil liberties," said former GCHQ official Bassett.
"It looks a lot harder to spot a single renegade individual than a criminal gang with a regular pattern of working."
Once dangerous material has been published on the Internet, few believe there is any reliable way of censoring or restricting it to curb its spread.
That is something Breivik would have been counting on when he dumped his 1500-page "manifesto", which not only incites further attacks but gives details of how he acquired weaponry and built his bomb, online shortly before the massacre.
Give the growing threat, Sara Silvestri-- an expert on radicalism at London's City University -- says authorities need to broaden their gaze further still when it comes to countering extremism, trying to overcome divisions and tackle the social isolation that helps create such potentially risky individuals.
Such an approach should also include mainstream politicians, educators and mental health workers, she said.
"Rather than deradicalization programs, I'd prefer to see 'living together programs'," she said. "The problem is that people do not know how to live and respect each other as human beings."
(Additional reporting by Sara Webb in Amsterdam and Michael Holden in London, Editing by Sonya Hepinstall)