A Quebec man who used a car to run down two Canadian soldiers, killing one and injuring another, had been on an RCMP watch list for months as a possible Islamic extremist and his passport had been revoked when he tried to fly to Turkey in July.

The Mounties tried to explain on Tuesday how Martin Couture-Rouleau evaded their grasp, saying they had arrested and questioned the 25-year-old Quebecker at the airport in July, but released him because they had insufficient grounds to lay charges. Turkey, his planned destination, is a gateway for jihadis bound for Syria.

RCMP brass said police talked to Mr. Couture-Rouleau as recently as Oct. 9, but they insisted authorities had no clear indication that the man, whom they considered radicalized, intended to commit a crime in Canada or abroad.

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"We could not arrest someone for having radical thoughts. It's not a crime in Canada," RCMP Superintendent Martine Fontaine told a news conference on Tuesday afternoon.

The case of Mr. Couture-Rouleau, who was shot dead by police on Monday in Saint-Jean-sur-Richelieu, Que., after the two soldiers were run down, illustrates the challenge facing authorities as Canada goes to war against Islamic jihadis in Iraq. The Canadian government appears better equipped to prevent would-be extremists from heading abroad to fight than thwarting them here, especially lone-wolf actors.

RCMP Supt. Fontaine said it is hard to stop domestic extremists who have the civil liberties enjoyed by all Canadians without solid evidence of a threat. "Unless we have clear indications of what he was doing, it was very difficult to prevent and stop him," the Mountie said.

A source familiar with the matter said police are investigating whether the suspect tried to reach out to Islamic State militants or other terrorist groups.

Police said on Tuesday they believe Mr. Couture-Rouleau set out to target soldiers, spending at least two hours in a Quebec parking lot south of Montreal before hitting the victims.

Warrant Officer Patrice Vincent, 53, died on Monday evening from his injuries. He had served in the Forces for more than 28 years from Victoria to Halifax. The other soldier, so far unidentified, is expected to recover.

RCMP said on Tuesday that, earlier this month, Mr. Couture-Rouleau gave police the impression he was changing his ways. "The meeting ended on a very positive note, so we had no reason to believe after he would commit a criminal act in Canada," Supt. Fontaine said.

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Officers in a patrol car who saw the attack chased Mr. Couture-Rouleau's Nissan Altima, and he was shot dead in a confrontation after he wielded a knife, police said. During the pursuit, Mr. Couture-Rouleau managed to dial 9-1-1 to claim responsibility for his act, the Sûreté du Québec confirmed on Tuesday.

At their press conference, the RCMP detailed the challenges they faced in preceding months trying to stop Mr. Couture-Rouleau from committing illegal acts.

"We didn't know his intention to use his car as a weapon," Supt. Fontaine said. "It would have been very difficult to prevent that. It's not a crime either to drive a car or be in a parking lot."

Mr. Couture-Rouleau converted to Islam last year and called himself Ahmad Rouleau.

The RCMP was alerted to him in June. "Some items on his Facebook page indicated that he was radicalizing himself" and was planning to travel overseas, Supt. Fontaine said.

The authorities designated him as a high-risk traveller, and he was among about 90 suspected extremists the force was investigating.

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When he tried to leave Canada in July, the RCMP arrested him.

However, the Public Prosecution Service determined it did not have enough evidence to press charges.

Authorities continued to investigate, and this fall met several times with him, his family, the imam of his mosque and outreach police officers.

During a long discussion on Oct. 9, he "show[ed] some intention of wanting to change and improve," Supt. Fontaine said.

She said Mr. Couture-Rouleau's behaviour showed nothing alarming in the days before the incident. "It's very hard when someone plans to do something alone, with little obvious preparations, and he uses a vehicle as his weapon."

RCMP Commissioner Bob Paulson said the Mounties do not believe Mr. Couture-Rouleau had partners, but are still probing.

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"We're open to that and we're concerned about that, so we're going to be pursuing every investigation avenue to satisfy ourselves that we've eliminated that possibility," Mr. Paulson told reporters.

"We are trying to gain a full understanding of the breadth and the reach that this individual might have had into other areas."

In the Commons, Prime Minister Stephen Harper praised Warrant Officer Vincent as a 28-year veteran who served with distinction."This was a despicable act of violence that strikes against not just this soldier and his colleagues, but frankly against our very values as a civilized democracy."

Since Monday's incident, it has emerged that the Integrated Threat Assessment Centre of the Canadian Security Intelligence Service, which warns government security agencies about terrorist threats, raised its warning level to medium from low last Friday.

Public Safety Minister Steven Blaney's office said the threat level change was not a result of the hit-and-run in Saint-Jean-sur-Richelieu.

"The increase is not the result of a specific threat," said Jean-Christophe de Le Rue, director of communications for Mr. Blaney.

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"The decision to raise the level is linked to an increase in general chatter from radical Islamist organizations like [Islamic State], al Qaeda, Al Shabaab and others who pose a clear threat to Canadians."

Also on Tuesday, eight CF-18 fighter planes left Cold Lake, Alta., for Kuwait as Canada prepares to join an international coalition launching air strikes against Islamic State militants.

Defence Minister Rob Nicholson said Monday's attack does not deter Canada.

"Our Canadian Armed Forces members represent the best of Canada, and to have one die in a senseless act such as this only strengthens our resolve," he said in a statement.

With reports from Colin Freeze, Les Perreaux and Ingrid Peritz