At least three Canadians who joined an al-Qaeda-linked militant group in Syria were directly involved in the holding and harsh interrogation of two U.S. journalists in the region, CBC News has confirmed.

Sources say the Canadians, whose identities aren't known by CBC, took part in the imprisonment of Theo Curtis and Matt Schrier, who were held in captivity together between 2011 and 2013.

Curtis was released by al-Qaeda affiliate group Jabhat al-Nusra last month, while Schrier escaped last year. Both men are still recovering and haven't yet fully detailed what happened in their years in captivity.

Theo Curtis

Theo Curtis, one of two journalists held and interrogated by Jabhat al-Nusra, was released from captivity last month. He later spoke to reporters outside his mother's home in Cambridge, Mass. (Charles Krupa/Associated Press)

The Canadian captors reportedly forced the hostages to hand over their computer passwords and PINs, draining their accounts. They then callously wrote letters and emails to the journalists' families pretending to be the men, and then set about going after their credit cards and racking them up, buying electronics and computers on eBay, according to the sources.

Those Canadians are believed to be alive and still at it — and their ranks seem to be swelling.

For the better part of a year, the Canadian government spoke of 130 being the rough number of Canadians fighting overseas with extremist groups, largely in Syria. That number is now thought to be vastly underestimating the total number.

From CBC's investigations, it now appears that the 200 to 300 range is more accurate.

There has been a recent upsurge in the recruitment of foreign fighters, the bulk heading to the murderous clutches of the Islamic State of Iraq and Syria (ISIS), also known as the Islamic State.

With files from Adrienne Arsenault