Poulin, from Timmins, Ont., converted to Islam and went by the name Abu Muslim. He joined the jihadist fight in Syria in 2012, in the process creating an 11-minute propaganda video for ISIS aimed at Westerners.
He died while fighting in northern Syria in the summer of 2013 at the age of 24.
CBC News has learned that Poulin also had a direct role in recruiting at least five young Muslim men from the Toronto area to go to Syria.
Four were rescued by family members before they could cross over from neighbouring Lebanon, but at least three appear to have returned there to join ISIS.
The first was Abu Turaab. His real name is Mohammed Ali of Mississauga, Ont. He left Canada in April 2014, eight months after Poulin was killed.
Experts monitoring Canadian jihadis believe that was Abu Turaab's second tour. They believe he left earlier, was injured in battle and returned to take advantage of Canada's health care.
His father enlisted an imam to talk sense into him, CBC News has learned. But the imam told CBC News that Abu Turaab was already determined to join ISIS.
What's surprising is that Abu Turaab and Poulin were friends on a popular online forum called Ummah.com as far back as 2009.
Poulin counselled Abu Turaab to earn some cash first before going to Syria and even to take out a $10,000 bank loan.
When he found out that Poulin had died, Abu Turaab tweeted: "Abu Muslim al-Kanadi [the Canadian]. I wouldn't be here today if it wasn't for him. May Allah accept him. Ameen."
Abu Turaab tweets are often so extreme that Twitter has had to shut down his account on numerous occasions. He has threatened Canada, praised the attacks on Parliament Hill and in Quebec late last year, and rejoiced at ISIS's beheading of foreign hostages.
Then on July 13, 2014, Abu Turaab tweeted this message. "As salaamu alaikum. This is the friends of Omar Abu Muslim from Canada. We are in Turkey now and we want to know which way to get into Syria and join Islamic state."
CBC News has learned the identity of these friends. They are Tabirul Hasib, Abdul Malik and two others, Adib and Nur, whose last names are not known.
Toronto's east end
The four are in their early 20s and are from the same neighbourhood in Toronto's east end. Born in Canada, their parents immigrated from Bangladesh to give their children a chance at a better life.
Hasib attended Monarch Park Collegiate and appeared to have graduated. He was active in his school's athletic program, participating in long-distance running competitions before enrolling in Centennial College.
Abdul Malik's father said his son never attended public high school, but was sent instead to an Ontario madrassa to memorize the Qur'an. As young boys they worshipped at one of several mosques in Toronto's east-end and participated in programs geared to Muslim youth.
Then their lives changed after Poulin, who was in his early 20s, showed up in 2011 wearing a religious turban and a long white tunic.
It didn't take long before he would became their imam and religious adviser. Sources told CBC News the four men met privately with Poulin once or twice weekly.
In late 2012, Hasib, Malik, Nur and Adib booked flights to the Middle East and disappeared.
Their panicked parents filed missing person reports with Toronto Police. Soon officers with the RCMP's Integrated National Security Enforcement Team were knocking on their doors and asking difficult questions.
Abdul Malik's father told CBC News that his son called home a few times after leaving to say he and his friends were safe and "in the course of the conversation revealed they were in Lebanon, not yet in Syria."
He discussed the urgency of the situation with another father and they realized they had to move quickly to rescue their sons.
The fathers of Abdul Malik and Nur got on a plane in February 2013 and headed to Lebanon. They met with their sons and convinced the four young men to return to Canada.
Abdul Malik's father said he asked his son why Poulin wasn't returning with them. The men said Poulin had decided to stay.
Poulin would soon enter Syria and join the ranks of Jabhat Al-Nusrah, an al-Qaeda front fighting to topple the regime of Bashar Al-Assad. He would be killed just a few months later.
Abdul Malik's father and the mother of another told CBC News that CSIS agents and the RCMP officers interviewed the four returnees several times after they came back.
The men withdrew from public eye and became isolated, if not agitated, according to people in the community who knew them well.
Some of them enrolled in community colleges.
Abdul Malik performed the pilgrimage to Mecca in late 2013. Their parents kept a close watch over them and the young men appeared to be adjusting to normal life once again, say elders in their community.
Still though, the parents took steps to hide their sons' passports. Hasib's mother told CBC News that she hid his passport and never imagined for a moment he would find it.
On July 6, 2014 three of the men, possibly all four, suddenly and without warning, disappeared once again. This time there would be no telephone contacts with their parents.
Unlike Abu Turaab, who is prolific on social media, the four young Canadian-Bangladeshi men have no known online profiles and it is not clear whether they are dead or alive.
Late last November, Tabirul Hasib called home. His mother recorded the date and exact time. "He said, 'hi mom.' I asked him where he was and he wouldn't answer and then I was overtaken by grief and couldn't speak to him," she said.
Abdul Malik's father and Tabirul Hasib's mother fervently believes someone "brainwashed" their sons and their friends.
They still don't have the faintest idea of the profound and chilly influence the man known as Abu Muslim has had on their sons.
For now, all they seem able to do is grieve and spend sleepless nights wondering whether their sons will ever return home.
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