The head of Canada鈥檚 spy agency says radicalized Canadians who have joined the Islamic State and other terrorist organizations overseas still pose a real threat to Canada鈥檚 security.

CSIS director Michel Coulombe says Islamic State recruits from the West could represent an even greater danger to their countries when they return home, as they may come back more radicalized and better-equipped to incite terror.

鈥淭he most obvious national security threat is the one posed by extremists who return,鈥 Coulombe writes in an editorial published by the Globe and Mail on Saturday.

He also says that even if a Canadian extremist does not come home, he or she still damages Canada鈥檚 international reputation.

While the vast majority of Canadian Muslims he鈥檚 worked with reject the Islamic State鈥檚 ideology, Coulombe says extremists have proven particularly effective at recruiting young people.

鈥淚t is now public knowledge that well over 100 Canadians have left Canada to support or train with terrorist movements abroad,鈥 he says. 鈥淢ost are men, but some are women. Some are immigrants to Canada, and some are Canadian-born.鈥

The Islamic State uses social media and videos to incite westerners to become soldiers in a so-called Muslim holy war. Coulombe says Islamic State recruiters try to 鈥渞omanticize the transformation鈥 of their recruits into religious warriors.

The organization has also been known to use western recruits killed in battle as martyrs for their recruitment videos.

鈥淔ortunately, most people will understand there is nothing romantic about killing and dying for such a cause,鈥 he says.

Coulombe pointed to the failed Toronto 18 terror plot as proof that Canada is not immune to attacks from extremist elements.

鈥淐anada must never assume that mass-casualty terrorism cannot happen here,鈥 he says.