Martin (Ahmad) Couture-Rouleau's passport was seized by authorities who feared he wanted to go overseas to take part in terrorism, blocking him from leaving Canada and highlighting a dilemma facing security officials dealing with the threat of militants on home soil.

Couture-Rouleau was arrested at the airport in July while on his way to Turkey, RCMP Supt. Martine Fontaine said at a news conference in Montreal.

He was identified as a high-risk traveller and had his passport taken away, but there wasn't enough evidence for police to charge him and detain him. Fontaine said police had several interactions with him after that and were in contact with his parents. Police spoke with him as recently as Oct. 9.

Couture-Rouleau, 25, was fatally shot on Monday after hitting two soldiers in a parking lot of a commercial plaza in St-Jean-sur-Richelieu, a city about 40 kilometres southeast of Montreal.

One of the soldiers, Warrant Officer Patrice Vincent, 53, died of his injuries Monday evening. Police said the other soldier doesn't have life-threatening injuries.

The case highlights the problem for officials who stop citizens from leaving Canada because of terrorism fears: those persons remain in Canada, perhaps with the intention of causing harm to others.

One more person to watch in Canada

RCMP Commissioner Bob Paulson touched on the problem Tuesday on Parliament Hill: asked whether it was possible Couture-Rouleau became more dangerous after his passport was taken away, Paulson said "certainly that's what follows from the analysis."

An officer with Quebec provincial police attends the scene of a police shooting in St-Jean-sur-Richelieu involving hit-and-run suspect Martin Couture-Rouleau. (Radio-Canada) "He was part of our investigative efforts to try and identify those people who might commit a criminal act of travelling abroad for terrorist purposes. So in that respect we were working him along with other suspects," he added.

A top official at the Canadian Security Intelligence Service, speaking at a Senate committee meeting Monday, put the dilemma more bluntly.

"For every individual that we prevent, every extremist that we prevent from going overseas to engage in extremist activity, is one more individual that we have to investigate closely because they're radicalized to the point that they want to leave," said Jeff Yaworski, CSIS deputy director of operations.

"There's nothing more that we can do with the budget that we have, except to prioritize internally as effectively as we can and I think we are doing that."

Yaworski said the agency's success rate "has been quite good."

"I'd be foolhardy to say that we have all the bases covered. We do what we can with the budget that we have, sir," Yaworski told senators.

The federal government has raised its internal threat level to medium due to an increase in "general chatter" from organizations like ISIS, but not because of a specific threat.

One of 90 people under investigation

Couture-Rouleau was one of 90 people being monitored by the RCMP as part of 63 current national security investigations, the RCMP confirmed to CBC News Monday night. Paulson first reported the investigations to MPs on the House public safety committee earlier this month.

That covered "both people who intend to go [abroad] or people who have returned and have been referred to us by the service," Paulson said on Oct. 8.

It included people suspected of being involved with extremism-related activities, including financing, not specifically to fighting alongside militants.

Legal documents show Couture-Rouleau converted to Islam in 2013. He went by Ahmad Rouleau on some social media sites.

Immigration Minister Chris Alexander, who has said his department is revoking the passports of those suspected of planning to travel abroad to commit terrorism, wouldn't answer questions as he left a cabinet meeting.

On Tuesday, Paulson said the RCMP are investigating along with Quebec provincial police to determine the "breadth" of this incident and pursue every avenue.

Paulson said the RCMP don't think Couture-Rouleau was connected to other sympathizers.

"We don't suspect that, but we're open to that and we're concerned about that. So we're going to be pursuing every investigative avenue to satisfy ourselves that we've eliminated that possibility," he told reporters on his way out of the committee meeting.

'Despicable'

A Quebec police spokesman said the RCMP are handling the investigation into the suspect and his motivations.

Martin Couture-Rouleau, pictured holding the Qur'an, had changed the name on his Facebook account to Ahmad the Converted. (Facebook) ​On Monday, the RCMP said the integrated national security investigations team in Montreal, along with other authorities, worried Couture-Rouleau "had become radicalized."

The Prime Minister’s Office issued a statement Tuesday, reiterating that federal authorities had confirmed certain elements clearly indicated Couture-Rouleau had become radicalized. Canadians should remain vigilant, the statement read.

"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," Harper said in the House.

Foreign Affairs Minister John Baird suggested there may not be more that law enforcement officials could have done, praising their work.

"I'm trying to think though what you could possibly could have done to stop someone who has not been arrested or not been accused of any specific criminal offence," Baird told Evan Solomon, host of CBC News Network's Power & Politics.

"You can't stop him from getting into a car. And I don't know whether, had he been under full surveillance, that that would have had any impact. We've got to be realistic as to what we can and cannot do. The fact he was someone who had radicalized but had yet to commit any criminal offence is important."

'Terrorist ideology'

Public Safety Minister Steven Blaney, speaking in St-Jean-sur-Richelieu Tuesday, said what took place "is clearly linked to terrorist ideology."

"This is a terrible act of violence against our country, against our military and against our values," Blaney said.

Chief of Defence Staff Tom Lawson said the safety and well-being of Forces members "is a primary concern."

"Security measures are in place at every Canadian Armed Forces installation across Canada. We continually adapt these measures to meet the demands of an evolving security environment, and we will remain vigilant against possible threats," Lawson said.

Defence Minister Rob Nicholson said in a statement that he felt "tremendous sorrow" over Vincent's death.

"Our CAF members represent the best of Canada, and to have one die in a senseless act such as this only strengthens our resolve. We will not forget," he said.