Michel Coulombe, director of the Canadian Security Intelligence Service, told a Senate committee the individuals' activities include paramilitary operations, training in weapons and explosives, logistical support, terrorist fundraising and studying in extremist schools.
Two cases of young Canadian men who died in hostilities in Syria recently grabbed headlines.
Coulombe says some never achieve their intent and simply return home, potentially posing a more immediate threat to Canadians.
He says CSIS actively investigates such people, but the probes are challenging, resulting in gaps in the spy agency's understanding.
Coulombe adds that the number of Canadians overseas is in constant flux, their motivations are difficult to ascertain and their movements across isolated terrain are difficult to track.
CSIS says the problem is not unique to Canada, and there is no simple solution.
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