The flow of aspiring fundamentalist fighters from the West to Syria is accelerating as that conflict becomes a global epicentre for self-styled jihadists.
"They are going in great numbers," said a former federal official, adding that there has not been a migration like this since Afghanistan in the 1980s. Overall, he said, "you're talking hundreds and hundreds."
The flow of would-be rebels – scores from Europe and the United States, and dozens from Canada – is said to be double the number given in recent assessments by intelligence agencies. For security agencies, this dynamic is heightening concerns about what will happen when these citizens return home as hardened warriors.
More than 120,000 Syrians have died in the country's civil war, which started with uprisings two and a half years ago.
President Bashar al-Assad, who stands accused of committing atrocities against his own population, hails from a minority sect in Syria, but presides over a majority Sunni Muslim population. This gives the civil war the sheen of a religious struggle for many of its participants, given that some fighters regard him as a heretic.
There is no consensus on the number of foreign fighters in Syria, but a British think tank this spring estimated that between 2,000 and 5,500 may have gone over.
The Canadian government has given no public estimate on the number of citizens who have joined.
There have been several accounts of Canadians fighting or dying. In February, unverified reports suggested a 24-year-old from Quebec was killed in a clash with the Syrian government.
In June, a mother in Alberta called The Globe and Mail to say federal security agencies had been inquiring about her son. She describes him as an Islamic convert who had been swayed to go to Syria by a network of recruiters in Western Canada.
"When I talk to him on the phone I hear planes, I hear gunfire, it's terrorizing," the mother said, asking to remain anonymous.
She added that her son told her he went to the city of Aleppo because he wanted to end "the bloodshed and torture that was going on" in Syria.
Last week, it was revealed that Ali Dirie, a Canadian of Somali descent previously convicted of terrorism offences in Canada, reportedly died fighting in Syria.
According to published reports, he may have fallen into an al-Qaeda affiliate known as the Al Nusra Front.