Authorities have revealed that Zehaf-Bibeau had a criminal history and that he had changed his name from Michael Joseph Hall on becoming a convert to Islam. He had also had his passport confiscated because he was believed to be trying to leave Canada to join a jihadist group in Syria. Just two days before his assault, another man known to police drove a car at two soldiers walking in a shopping mall in a town south of Montreal, killing one them before in turn being killed by police. Like Zehaf-Bibeau, Martin Couture-Rouleau was a Muslim convert who had had his passport removed by authorities after being identified as a likely volunteer for IS.

Remarkably, Canada's domestic terror threat level appears to have remained at low after the hit-and-run in Quebec, and to have been raised to medium on Tuesday only after "an increase in general chatter from radical Islamist organisations". A 2012 recommendation for greater security at the Canadian Parliament was never implemented, despite the fact radicalised young Canadians have figured in some of the most notorious terrorist incidents of the past two years, including the storming of the Westgate Mall in Nairobi and the attack on a southern Algerian gas plant by an al-Qaeda offshoot.

The immediate question facing Canadian authorities is whether Wednesday's attack was inspired by the hit-and-run incident or was part of a coordinated plan for continuing terror attacks. Changes will need to be made to better secure public buildings and to protect politicians and military personnel. There will be plenty of wider soul-searching too, and not just because of the striking similarities between the two attackers. This was week when the Harper government sent six fighter jets to Kuwait to join the US-led coalition striking IS positions in Iraq, and many will wonder whether this inspired the two men. Some will ponder the wisdom of Mr Harper's aggressively anti-Islamic State posture since the group came to prominence, a stance which earned Canada special mention in the fatwa issued last month by Abu Mohammed al-Adnani. "If you can kill a disbelieving American or European – especially the spiteful and filthy French – or an Australian, or a Canadian or any other disbeliever, then rely upon Allah, and kill him," the IS spokesman said. "Smash his head with a rock, or slaughter him with a knife, or run him over with your car".

IS's sinister ideology, its brutality on the battlefield, and its unexpected success in asserting control over large areas of Iraq, have provoked an understandably strong reaction from the West. Preventing young, radicalised Muslims from setting off to the Middle East to join IS's fight has been one among many responses. But how much safer will it make us? In stripping these men of their passports, have countries like Canada (and Australia) merely closed off the option for violence abroad and left open the choice of violence at home, a path some seem only too eager to take?

The potential for young men and women to fall under the sway of propagandists is well understood. How to lessen the risks of that occurring is more difficult to determine. One thing seems evident in the wake of the Ottawa attack: simply stripping people of their passports on suspicion they might venture to a war zone is no solution on its own. It may merely see the trade of a dusty battlefield abroad for an attack at home. If there are indeed grounds for the likes of Australia or Canada to confiscate their citizen's passports, would ensuing criminal charges be the best way to eliminate that threat?

It's a question deserving of as much close attention here as in Canada.