The RCMP say there was little they could have done to prevent Martin Couture-Rouleau from targeting two soldiers in Quebec, despite their monitoring of his activities, his recent arrest, and subsequent attempts to de-radicalize him.

Supt. Martine Fontaine, who heads national security for the RCMP in Quebec, told reporters that Mounties arrested Couture-Rouleau when he tried to fly to Turkey in July.

The 25-year-old was later released, Fontaine said Tuesday, because there was not enough evidence to prove he was planning to fight alongside terrorists. His passport was seized, but Fontaine said there was little else police could do.

"It's difficult to do more, because we could not arrest someone for having radical thoughts, it's not a crime in Canada," she said. "And unless we have clear indications of what he was doing, it was very difficult to prevent and stop him."

Fontaine also said there was no evidence that Couture-Rouleau was planning an attack in Canada.

Following his arrest, RCMP officers reached out to his family and the imam of a mosque he attended in a bid to de-radicalize him. They met with Couture-Roleau on Oct. 9, at which time he suggested he was interested in changing his life, Fontaine said.

But on Monday, less than two weeks after that meeting, Couture-Roleau allegedly struck two members of the Canadian Forces with his car in St-Jean-sur-Richelieu, Que.

After a short police chase, he was shot and killed by officers. Police said Tuesday that he had called 911 after the attack to claim responsibility for the hit-and-run.

One of the soldiers, identified as 58-year-old Patrice Vincent, died from his injuries, and the second soldier suffered minor injuries.

Attack would have been 'very difficult to prevent': RCMP

Police said they initially began investigating Couture-Rouleau in June, after his social media posts suggested he was "radicalized to the idea of travelling overseas."

RCMP Commissioner Bob Paulson said Tuesday that the suspect was among 90 suspected extremists living in Canada that the Mounties are actively monitoring.

But Fontaine said RCMP officers stopped monitoring Couture-Roleau after the Oct. 9 meeting, but remained in contact with his family.

She said that the attack on the soldiers would have been "very difficult" to prevent, and there was no indication he was planning on using his car as a weapon.

"It would have been very difficult to prevent that… because it's not a crime either to drive a car or be in a parking lot," she said.

University of Calgary's Prof. Robert Huebert, who researches Canadian foreign and defence policies, said that police have few options in cases like the one in Quebec.

"Ultimately we are in a free and open society, and if you want to have radical ideas, that in and of itself is not against the law," he said.