While counter-terrorism agencies have disrupted international mass-casualty plots by the Toronto 18 and Via Rail conspirators, the unsophisticated loners have gone unnoticed until it was too late.

Terrorist groups like ISIS are well aware that lone attackers using simple weapons are more difficult to detect, which is why their propaganda has encouraged supporters to kill with knives, vehicles and even rocks.

“It’s nearly impossible to stop these opportunistic attacks, particularly from lone wolves that don’t have a digital footprint per se, or aren’t part of a larger organization,” said Colin Clarke, a senior research fellow at the Soufan Centre.