With a man in custody for what Edmonton police call "acts of terrorism" in the city overnight, investigators will try to determine if he had associates who aided him or could pose a continuing danger to the public, says a Canadian security expert.
The 30-year-old suspect was apprehended following a high-speed chase just before midnight through streets filled with bar patrons and football fans that ended only after a white U-Haul van he was driving struck four pedestrians and flipped on its side.
"One of the difficult questions that will be asked here is: what if anything did security intelligence and criminal intelligence know about this individual beforehand? Was he under surveillance?" Christian Leuprecht, a political science professor at the Royal Military College of Canada and Queen's University, told CBC News on Sunday.
"And if he was, whether there would have been enough evidence to charge him before this attack would have taken place. Those are difficult questions, and those will all come out during the course of the investigation."
Leuprecht, who is currently attending a security conference in Latvia, said Public Safety Canada tries to track extremists who travel to Syria or Iraq to try to join ISIS.
"We also know that there are between 80 and 120 of those individuals who have so far returned to Canada," he said. "Some of these individuals remain under surveillance for as long as six months. So it would be interesting to see if [the man arrested in Edmonton] had a previous travel history to the region or connection with someone else who did."
Security expert Michel Juneau-Katsuya said the evidence suggests the suspect was an ISIS supporter.
"A flag was found," said Juneau-Katsuya, CEO of security consulting firm The Northgate Group. "The methodology that he used was very, very similar to methodology used before … and we knew that there [are] still some supporters or sympathizers of the Islamic State right here in Canada.
"We were expecting that it would happen in Canada at one point or other. We just didn't know when or … where."
Juneau-Katsuya said it is likely the attacker was inspired by ISIS but "very unlikely" he was directed by the jihadist organization.
"This is the kind of incident by individuals that can be taken on their own initiative, and this is what is so terrible and so difficult to investigate for police officers because basically you need no training.
"You just need the conviction and the commitment to do whatever horrible attack you want to do, and that's it. You find a vehicle. You find a knife. You find your victims."
Vehicles have been used as weapons in several deadly attacks around the world in recent years.
In August, a van drove along sidewalks in a popular tourist area in Barcelona, killing 13 people and injuring more than 100.
In June, seven people died and four dozen were injured after a speeding van mowed down pedestrians on London Bridge. Three men with knives later abandoned the van and attacked people in nearby Borough Market, an area full of bars and restaurants.
In December 2016, a truck plowed into a crowded Christmas market in Berlin. A dozen people died and many more were injured.
The deadliest such attack in recent years happened in Nice, France, where a large truck plowed through revellers gathered for Bastille Day fireworks on the waterfront promenade, killing at least 84 people. A student who attended MacEwan University in Edmonton was among the dead in that attack.