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Yearbook photo of Aaron Yoon. London South Collegiate Institute

In 2008, Aaron Yoon, the London, Ont., man who has been reported to be in an African jail under mysterious circumstances, was a "fun-loving" teenager who worked part-time at a local restaurant and hung out with friends from work. But according to a former co-worker, something changed in the Korean-Canadian teen over the next few years. She watched her former friend become increasingly isolated and religious. "My Facebook was flooded with posts about Islam … It was getting excessive so I just deleted him," the young woman, who asked not to be identified, said.

The next time she saw him, three years after they first met, was at another workplace where he happened to be hired at the same time. "He looked like he was a shell. He was empty," she said.

By then, she said, he was praying several times a day and refusing to talk to his female supervisors.

Late Tuesday, the CBC reported that Mr. Yoon travelled to Africa with Xris Katsiroubas and Ali Medlej. Mr. Katsiroubas and Mr. Medlej are both implicated in an al-Qaeda-inspired conspiracy that killed 38 Westerners – and the both of them – at an Algerian gas complex in January.

Mr. Yoon was arrested some time before the attack, and there is no evidence that he participated in planning it. The man's family denies he has links to terrorism or that he is in jail, the CBC reported.

The three men met while attending London South Collegiate Institute together. Mr. Medlej graduated in 2006 and Mr. Katsiroubas left after Grade 11. Both Mr. Yoon and Mr. Katsiroubas are believed to have converted to Islam while attending high school together, with Mr. Katsiroubas changing his name to "Mustafa" in Grade 9, according to the London Free Press.

How they journeyed from a Southwestern Ontario city to the front lines of an attack that shook the world has not been revealed. But federal security officials have openly dreaded just such a "homegrown" terrorist event for more than a decade – arguing that it was always a question of when, and not if, a Canadian radical would commit a massacre.

This fear now has proof and human faces. Canadian police and intelligence officials have been to Algeria in recent weeks, obtaining DNA that confirms the Canadians' involvement.

Security officials won't officially comment on the identities, but the names of Mr. Medlej and Mr. Katsiroubas were revealed by the CBC this week.

Now the RCMP and the Canadian Security Intelligence Service are trying to figure out who the men's accomplices were, even as they retrace past investigations. Years before, the two men had been red-flagged by authorities as potential threats to Canada, along with members of their wider circle.

The CBC also reported that Mr. Medlej and Mr. Katsiroubas moved together to south Edmonton in early 2007.

"Ali was a smooth talker. He was definitely the leader. Xris was quiet," their landlord was quoted as saying. The CBC reported that Mr. Medlej was fined $500 after pleading guilty to shoplifting groceries that March and that they were evicted for damaging their apartment.

It was around this time that the pair, as well as members of their wider circle, first appears to have fallen under scrutiny by the Canadian Security Intelligence Service – an agency constantly pressed to make hard decisions about whom to keep tabs on, and when to cease surveillance.

Ray Boisvert, a former assistant director of intelligence, said he could not comment on specific cases, but his work involved hard choices. "I sucked my breath back many times, to go 'Okay. Drop them. Move to these people,' " he said.

In London, a local mosque held a news conference Tuesday saying that the two men were unknown to members of the prayer centre. RCMP and CSIS officials visited the same mosque last month to encourage young Muslims to report extremism to authorities.

With a report from Josh Wingrove in Edmonton

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