They pulled up to an Edmonton bus terminal, their Southwestern Ontario childhood homes now more than 3,000 kilometres in the rearview mirror. Like so many Canadians who came before and after them, the pair of teenagers travelled west to Alberta with the hope of a decent job, a chance at prosperity, a shot at a better life.

In 2007, Xristos Katsiroubas and Ali Medlej left London, a manufacturing hub with disappearing jobs. Boasting little in savings, they turned to an elementary-school friend, who fronted them money for a deposit on an apartment and drove them around the city handing out résumés. Their dream of achieving some measure of financial stability was met with the reality of employment at places like Applebees, as a cook, and as a gas-station attendant. Those gigs didn't last long.

Within months, Mr. Katsiroubas, a high-school dropout and convert to Islam, and Mr. Medlej, the elder of the two by a year, found themselves embittered, discouraged by a string of setbacks and confronted with a feeling that the world was conspiring against them.

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Somewhere along the line, in London or Edmonton or some other place yet to come to light, the deflation and annoyance morphed into something more than typical teenage angst: In January of this year, Mr. Katsiroubas and Mr. Medlej, both in their early 20s, were found dead in the middle of the Sahara after a terrorist attack on an Algerian gas plant that killed dozens of hostages. They were not victims.

What transformed the two from average teenagers to two men accused of helping organize a violent siege remains a mystery. Similarly murky is the case of a third London man, a friend named Aaron Yoon, who is being held in a Mauritania jail under suspicion he is a jihadi with the hardline Salafist movement.

The Globe and Mail went to their hometown in search of answers, interviewing dozens of friends, neighbours and former classmates, as well as making calls across the country and overseas. The families, for their part, have chosen to guard their privacy; only Mr. Yoon's older brother spoke out. The men's upbringings were painted by those who knew them over the years as mostly unremarkable.

But then, of course, something changed.

When Mr. Katsiroubas arrived in Edmonton with Mr. Medlej, he was just a regular teen, according to Basel Alsaadi, Mr. Katsiroubas' childhood friend from London who was already living in Edmonton. The two newcomers moved into an apartment with a third Londoner named Benjamin Thomas, having parties and girls over "pretty much every night," Mr. Alsaadi said.

It was not long, though, before Mr. Katsiroubas and Mr. Medlej realized they couldn't sustain their lifestyle. They resorted to stealing to get by, with court records reportedly showing that both Mr. Medlej and Mr. Thomas pleaded guilty to shoplifting in May, 2007, each slapped with a $1,000 fine. "They felt like others had privileges and that the world was unfair," Mr. Alsaadi said.

Some of those feelings stemmed from their views on politics. Mr. Medlej, whose parents are believed to be Lebanese, was overheard talking about Lebanese conflicts in the Middle East – particularly its war with Israel in 2006 – saying he felt his people were being taken advantage of, bullied even. "They had an overarching understanding that things were not fair," Mr. Alsaadi said.

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Their living situation was beginning to grate on them, too. Both Muslim, Mr. Katsiroubas and Mr. Medlej grew increasingly annoyed at Mr. Thomas' drinking and partying with friends at the apartment, Mr. Alsaadi said. Their landlady wasn't impressed either, and they were soon evicted.

Mr. Alsaadi never thought Mr. Katsiroubas was in any kind of serious trouble until the Canadian Security Intelligence Service approached him – tipped off, according to the CBC, by someone who knew them. "It was alarming," Mr. Alsaadi said. "I actually told them, 'What are you guys up to, what's going on here?' "

Mr. Alsaadi, who hadn't had contact with either of the men in years, said he really doesn't think the two were planning anything at the time. He feels, though, that the experience emboldened them. "The fact that they were being questioned kind of gave them that self-importance," he said. "It kind of made them be even more rebellious."

Residents in the men's hometown were aghast at this week's revelation that two of their own were implicated in the North Africa attack and that Mr. Yoon had been detained. The news is a conversation starter among strangers holding newspapers on street corners; it is on the lips of neighbours, friends and relatives trying to reconcile the portrait of the families they knew years ago with the narrative emerging now about the three men.

For Mr. Alsaadi and another childhood friend, the bearded, Muslim-garbed Mr. Katsiroubas, who ultimately went by the name of "Mustafa," was wholly unrecognizable next to the memory of a Greek boy named Xris. The young Xris played volleyball, basketball and road hockey. He went to the movies and visited a Greek bakery with his father, Stan, and he was a "Conflict Cooler" who helped settle playground spats at St. George Etienne Cartier elementary school.

The school, in a well-to-do neighbourhood, is a walk away from the house he called home after his parents divorced. His mother moved into the Chiddington Avenue home, already owned by her new husband Ian Hawn, merging the two families. "It was a Brady Bunch situation," said the childhood friend's father, who hosted Mr. Katsiroubas for playdates.

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Mr. Alsaadi remembers feeling as though Mr. Katsiroubas' mother and Mr. Hawn, his engineer stepfather, played favourites. With the two families under one roof, Xris was one of three boys, alongside his biological brother Andrew and a stepbrother, Eric. Mr. Katsiroubas often descended into the basement alone to play video games.

Mr. Katsiroubas and Mr. Yoon went to elementary school together, but since Mr. Medlej attended a different school, he didn't come into the fold until high school at London South Collegiate Institute – until Grade 10 or so, when he and Mr. Katsiroubas "clicked," as one former schoolmate put it.

Mr. Medlej was a year older and not like the other two. While Mr. Yoon and Mr. Katsiroubas were described in their early high-school years as inconspicuous, Mr. Medlej was remembered for an oftentimes "jolly" nature, a sometimes hot temper and his impressive smarts.

"In my memory, he's got a smile and kind of happy-go-lucky [personality] for the most part," said former classmate Michael Melito, who played football with Mr. Medlej. He added, though: "[He was] quick to anger, but not an angry person … Maybe someone who had a shorter fuse."

One friend remembers Mr. Medlej telling him he took a baseball bat to a "group of white kids" who were yelling at him from their porch. Another former schoolmate said he got into a brawl with Mr. Medlej outside a London bar in 2008 – there had long been bad blood between the two, the schoolmate said, since he was convinced Mr. Medlej stole his backpack in high school.

But the friend said Mr. Medlej was also incredibly bright. "He and I would sit together [in class], and goof off or whatever, but I would get mediocre marks, whereas he would goof off and still get like 90s," he said of studying together at the collegiate, which is in one of the city's oldest neighbourhoods.

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Although this friend saw Mr. Medlej as a leader who didn't travel with a pack, he believed Mr. Medlej wanted to "fit in" with the Lebanese cohort at a school somewhat divided between the "white" students and an immigrant population captured by the school after it widened its net to the south.

It is there, in the south London neighbourhood of White Oaks, that Mr. Yoon's Catholic parents live. Unlike the Katsiroubas house, with its healthy lawn and backyard pool, the Yoon apartment is on the dimly lit third-floor of a three-storey walk-up condominium complex across from a mall.

By 2006, Mr. Katsiroubas had left behind his Greek Orthodox upbringing for Islam, praying daily with Mr. Medlej at the high school, one former schoolmate said. Mr. Katsiroubas started going by the name "Mustafa," sporting a long beard and then, after dropping out of school around Grade 11, donning more traditional Muslim clothing.

"Maybe he just found it difficult, maybe he didn't have the support," Mr. Alsaadi said. "Maybe it was the people he hung around with didn't value school, per se, and wanted to just move on and do other things."

Indeed, the friend who grew up with Mr. Katsiroubas said "Mustafa" started hanging out with a different crowd, all but ignoring this former friend at school. "Right away, he changed [after converting]," he said. "He travelled many places and came back with a beard. You couldn't recognize him. He started wearing different clothes. You'd see him in the smoker's pit and he wouldn't say a word to anyone."

The nature of any travel outside Canada is a major question mark in this evolving tale. It may be what prompted CSIS to ask the RCMP to put Mr. Katsiroubas and Mr. Medlej on their radar two years ago, a move that suggested law-enforcement authorities were increasingly concerned about their potential crimes.

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When threats are felt to be serious enough, CSIS sends a "disclosure letter" or an "advisory letter" to the RCMP concerning the most dangerous extremists. The disclosure letters provide tips and leads to the Mounties, while advisory letters provide the RCMP with information that is intended to support police searches, wiretaps and, eventually, criminal charges.

Little is publicly known about where Mr. Katsiroubas had journeyed before meeting his death at the Algerian gas plant in January. Mr. Alsaadi said he believes Mr. Katsiroubas and Mr. Medlej left Canada in 2009 – for where, he doesn't know – but the pair must have returned before venturing to North Africa because they were both spotted in London in the years between.

The friend who went to elementary school with Mr. Katsiroubas said he saw Mustafa in early 2012, shortly after one of Mr. Katsiroubas' high-school friends, Said Hadbai, was shot dead after he reportedly stepped in to help a friend involved in a fight. The friend said hello and offered his condolences; Mr. Katsiroubas, whom the friend said was "dead to [the family]" by this point, offered but a nod.

Mr. Medlej, for his part, got married sometime in late 2009 or early 2010. A friend posted a message to Facebook on Jan. 9, 2010, saying his "bro Ali Medlej" had just got married. The day before, the friend wrote that Mr. Medlej was planning to "hang" with him and Mustafa in London – that they were going to "the mosque."

A year later, Mr. Medlej was working at a downtown London Hasty Market.

Mr. Yoon came to the Muslim faith later than Mr. Medlej and Mr. Katsiroubas, converting from Catholicism to Islam three or four years ago, his older brother told The Globe. One woman who said she worked with Mr. Yoon while he was a busboy at a Mandarin restaurant in 2008, and then again when he hauled furniture from the stockroom at a Leon's warehouse in 2011, noticed a major shift in those years.

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At Leon's, he prayed daily and refused to converse with female supervisors since, he said, women had no business being "superior." He flooded his former coworker's Facebook feed with so many quotes from the Koran that she ultimately deleted him from her social network.

"He was this fun-loving guy at the Mandarin," she said, adding that he used to have after-work drinks with coworkers and spend time with a girlfriend. "And then when I saw him at the Leon's, it was a complete shock. He isolated himself from people and hung out alone."

One of Mr. Yoon's two older brothers, who answered the door Wednesday at his parents' White Oaks dwelling with tired, red eyes, said Mr. Yoon became a "better person" when he adopted the teachings of the Koran, fostering a better relationship with his family.

Around the time Mr. Yoon is believed to have converted and the other two returned from Edmonton disgruntled at an "unfair" world, two Canadian diplomats – Robert Fowler and Louis Guay – disappeared while on a mission in Niger. They were held hostage for four months by the men of Mokhtar Belmokhtar, a leader of al-Qaeda in the Islamic Maghreb (AQIM) and the man believed to be behind the Algeria hostage-taking.

Mr. Yoon's brother firmly rejects the notion that Mr. Yoon had become radicalized – or that he journeyed to North Africa for any reason other than to join friends, initially in Morocco.

It turns out that Mr. Yoon left for Mauritania in May, 2011, to study Arabic in the capital, Nouakchott, according to Amnesty International. Just seven months into his African trip, he was arrested as a terror suspect, and by July, 2012, he was sentenced to two years in jail and a fine, said an Amnesty International researcher, Gaëtan Mootoo.

News of this third Londoner's detention went unnoticed because it was published in a little-known Arabic-language publication. In a July 29, 2012, article, the Mauritanian magazine El Hourriya reported that a Canadian named Haroun Yoon was in custody on suspicions that he was a jihadi with the radical Salafist movement.

That summer, Mr. Mootoo visited Mr. Yoon, but the Canadian said he didn't want the human-rights group to lobby on his behalf. "He didn't want Amnesty to campaign for him so I have to respect that," Mr. Mootoo said in a telephone interview Friday from Paris.

The fate of Mr. Yoon's friends took longer to emerge. They had ended up with the militants of Mr. Belmokhtar, who had kidnapped the Canadian diplomats four years before. Having splintered from AQIM, Mr. Belmokhtar's group now called themselves the Al-Mulathameen brigade and "Those Who Sign with Blood."

Travelling from northern Mali to Algeria via the Libyan desert, the two Canadians and other militants arrived at the In Amenas gas field just before dawn on Jan. 16. They fired at two buses of workers, then stormed the living quarters, dragging out workers, targeting Westerners, killing those who tried to escape and rigging others with explosives.

The following evening, a member of Those Who Sign with Blood called a Mauritanian news agency, Agence Nouakchott d'Information, and revealed that the terrorist commando included militants from Canada. The bloody raid ended with a standoff with Algerian security forces and the death of 38 hostages and 29 terrorists.

Survivors told reporters one of the terrorists had fair hair and spoke fluent English. While the Algerian prime minister quickly alleged that two Canadians played a key role in the hostage taking, it would only be in March that the RCMP confirmed their nationality and this week that the police force acknowledged Mr. Medlej and Mr. Katsiroubas had died there.

For Mr. Yoon, freedom may come by the end of this year since his sentence is up in December, Mr. Mootoo said, adding that the Canadian is being held in relatively decent surroundings, has regular meals and is able to pray.

In an interview Friday night with the CBC from his cell in Mauritania, Mr. Yoon denied the allegations against him and said he was shocked at the news his friends were involved in the attack. He also complained of maltreatment, saying, "I've been beaten, I've been tortured, and I'm still sick, and I still don't see any medical attention, I don't see any help from the Canadian government in my release."

Mr. Yoon's older brother said he spoke with him many times over the phone, as recently as last weekend, when he gained assurances that "everything was fine."

"I hope Aaron comes back and clears all of this up," he said. "We're waiting for his call."

With reports from Colin Freeze, Rich Cash and Daniel Leblanc