As police work to clear the scene of a foiled terrorism attack in Strathroy, Ont., residents are questioning law-enforcement's handling of the case and asking why they were not informed that an Islamic State sympathizer was living in their community.

Forensic officers and police dogs continued to comb the property where 24-year-old Aaron Driver had been living under a peace bond until he died Wednesday, in a confrontation with the RCMP that culminated in a bomb blast and gunfire. Canadian officials had acted on a tip from the FBI, which had somehow become aware of a video showing a man pledging allegiance to IS leader Abu Bakr al-Baghdadi.

Within hours, the RCMP determined that man to be Mr. Driver. A tactical team descended on the home and intercepted the bomb-carrying man as he was about to leave in a taxi, bound for a mall in downtown London, Ont. Although Mr. Driver was the subject of a peace bond, he was not under physical surveillance – a reality that underscores the challenges police face in keeping tabs on people they fear may carry out terrorist activities but who have not yet committed an offence.

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At a news conference Friday, alongside the mayor and the local police chief, RCMP Superintendent Jamie Jagoe called Wednesday's police effort an "incredible example" of how international partners collaborate to fight terrorism. Asked about why the FBI, and not the RCMP, found the video, he said information flows both ways and that this time, it flowed to Canada. The FBI would not say how it became aware of the video, telling The Globe and Mail in an e-mail that it must protect its "methods and techniques."

Supt. Jagoe and Strathroy-Caradoc police Chief Laurie Hayman urged Canadians to be vigilant and report any suspicious or unusual behaviour to their local law-enforcement agency. But at least one of Mr. Driver's neighbours did just that, only to be left wondering what, exactly, the police had done to ensure there was no threat to public safety.

Neighbour Maria Pereira said she called the local police on July 31 after she heard what sounded like firecrackers coming from the backyard. The police drove by, she said, but she does not remember them going to speak with the resident. Asked about the police response to that call, Chief Hayman said she could not comment because of the ongoing investigation.

Residents on the quiet residential street where Mr. Driver had been living with his sister were horrified to learn that a known IS sympathizer had been living in their midst. Several said that had they known, they would have kept a closer eye. "The police knew about him," neighbour Colin Meade said. "They should have made it known that this guy was a [potential] terrorist … It will be a long time before this is forgotten."

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Strathroy-Caradoc Mayor Joanne Vanderheyden told The Globe she had known for several months that a "person of interest" might end up in the town, but she did not know whether the individual was male or female, let alone his name.

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Mr. Driver, whom authorities now believe was planning to detonate a bomb in a Canadian urban centre, had been arrested in June of 2015 because of his online activities. Even though he had been in contact with British IS jihadis and a gunman who was later killed during a terror attack in Garland, Tex., there was not enough evidence to warrant criminal charges, so the Crown sought a peace bond. While waiting for a court date, he was under bail conditions that included a tracking bracelet and reporting weekly to the RCMP.

In February, he agreed to a peace bond that barred him from having a cellphone or a computer, but no longer required the electronic bracelet. He was to report twice a month to an RCMP officer in London and live with his sister in Strathroy. Asked why police did not alert the community to his presence, Supt. Jagoe told reporters there is nothing in Canadian legislation that would allow police to issue a public warning about someone released into a community on a peace bond.

"Quite frankly, I don't think anybody dropped the ball on this particular case," he said.

In recent years, attacks involving suspects acting without accomplices have popularized the notion of "lone-wolf" terrorism, but what happened this week points at another dynamic: how to deal with "known-wolf" cases. These are people who have already been detected – who are already known as having radical views – but who remain at large because they have not committed a specific crime.

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In France just last month, one of the killers of a Catholic priest turned out to be Adel Kermiche, a known IS supporter who was under a court-ordered curfew and had been required to wear a tracking bracelet.

Patrick Skinner, a former CIA case officer, said today's international counterterrorism systems are straining to keep track of the increasing numbers of extremists. "With a large pool of known [or] suspected extremists, to say nothing of the ones totally off the radar, law enforcement and [intelligence] services must prioritize risk and coverage," said Mr. Skinner, who now works for the Soufan Group, which provides strategic intelligence services to governments and organizations.

Past testimony before Canadian parliamentary committees has shed some insight into the scope of the problem; it takes up to 20 officers to follow a single suspect, lawmakers have been told.

Earlier this spring, CSIS Director Michel Coulombe said the agency was aware of about 180 Canadians who are engaged with terrorist organizations abroad, while another 60 were back in Canada. RCMP Commissioner Bob Paulson said at the time that Canadian security agencies are keeping tabs on the 60 people who have returned here, even if there is not enough evidence to charge them.