Homegrown. Lone wolf. High-risk traveller. These words are now part of the lexicon of a renewed war on terrorism, a vocabulary Ottawa officials use as they grapple with extremism inside Canada's borders.

And these words describe Martin Couture-Rouleau, a 25-year-old Quebecker who, according to Canada's national police force, was one of 90 suspected Canada-based extremists who have lately been seeking to travel to Middle East war zones.

Police said Tuesday they had blocked Mr. Couture-Rouleau as a "high-risk traveller" last summer, keeping him grounded in Canada as he tried to fly to Syria. After that, police had considered him a man worth watching – yet Crown lawyers did not regard him as a candidate for a viable prosecution.

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Such is the case, far more often than not, in Canadian counterterrorism circles, where fears run high yet evidence of provable crimes is often lacking. Authorities say they took the precaution of seizing Mr. Couture-Rouleau's passport – an administrative measure once considered extraordinary, now more common as fears about the Islamic State rise.

"The number of individuals brought to our attention leaving the country started to increase significantly as the events in Syria started to unfold," Superintendent Wade Oldford told The Globe and Mail in an interview last month.

The country's top counterterrorism detective was explaining why a task force has been formed: to keep such suspects grounded, and minimize the risk they present.

Keeping extremists away from war zones is just one way to battle an increasingly hard-to-predict terror threat. But since the U.S.-led bombing coalition against Islamic State began, the group has been urging sympathizers abroad to hit back. "If you kill a disbelieving American or European … or an Australian, or a Canadian … kill him in any manner," an IS spokesman urged earlier this month.

In Saint-Jean-sur-Richelieu, Mr. Couture-Rouleau appears to have alienated many of his family and long-standing friends. He is not known to have had a circle of local extremist sympathizers around him.

"We don't suspect that," RCMP Commissioner Bob Paulson told reporters Tuesday. But he did confirm that the Quebecker is one of the people he had in mind when he recently told Parliament that the Mounties have 60 ongoing investigations into 90 suspected extremists who have travelled to join terrorist groups – or who have been thwarted trying to travel.

This statement means the RCMP, which arrested 18 terrorism suspects at once in Toronto in 2006, are now averaging 1.5 suspects for every such probe. This speaks to an increasingly diffuse dynamic where suspected extremists in Canada no longer congregate much – at least not in person.

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On the Internet, Mr. Couture-Rouleau, who was shot dead Monday by police after he ran over two Canadian Forces members, had called himself "Ahmad" or "Ahmad the Convert." His apparent Twitter account shows he followed a number of known firebrand preachers, including one based in the United States, Ahmad Jibril, and another in Britain, Anjem Choudary. Both of these men have been branded as radicalizers, yet they manage to stay within their free-speech rights.

The Islamic State continues to have thousands of low-profile supporters on Twitter, including an ostensible Canadian who, claiming to be in the so-called "caliphate," took note of this week's attack. "Muslims in Canada, follow the footsteps of our brave brother Martin Rouleau," the message said.