CBC News has learned that two Canadians linked to al-Qaeda and killed while staging a bloody attack on an Algerian gas refinery earlier this year were former high school friends in their early 20s, one from a Greek Orthodox family, and both from a comfortable middle-class London, Ont., neighbourhood.

A special CBC News investigation has confirmed the two al-Qaeda linked militants are Xristos Katsiroubas and Ali Medlej, both believed to be under 24 years old.

The attack by the two Canadians and 30 other militants linked to al-Qaeda left more than three dozen refinery workers dead, the final 10 of whom were reportedly tied to gas plant piping and killed in a massive bomb blast.

Sources say it is likely Katsiroubas and Medlej intentionally blew themselves up in the blast; only one of them could be identified by DNA testing.

- RELATED: 2nd Canadian confirmed killed in Algeria attack

The Harper government at first denied there was any evidence Canadians had been involved in the attack, and the RCMP have continued to keep the identities of the pair a tight secret.

Now sources tell CBC News there may have been more than just the duo.

Sources say at least two more former London schoolmates of Katsiroubas and Medlej also travelled overseas with them.

But it is not yet clear whether the others are still alive, nor if they were involved in the Algerian gas plant attack.

Police sources say Katsiroubas is the likely attacker whom survivors have described as blond-haired and speaking fluent "North American English."

Canada’s intelligence service is refusing to comment.

But CBC News has learned that as long ago as 2007, CSIS agents interviewed some family and friends of the two, who were then teenagers.

A former London friend of the two men said one of their relatives called the police, complaining they were "hanging around with weirdos."

He says a CSIS agent subsequently interviewed a number of their friends.

Not under surveillance

Intelligence sources say CSIS did not have the two men and their other friends under surveillance when they left Canada sometime last year on their ultimately bloody mission.

No one will ever know all the details of how and why two teenagers who seemed rather normal ended up committing such monstrous acts.

But the details that are emerging from former friends and associates suggest theirs is a story that is becoming frighteningly more common.

"Xris" Katsiroubas grew up in a home with a backyard swimming pool in a middle-class London neighbourhood with rich ethnic diversity.

He lived with his mother after his parents divorced, and by all accounts seemed normal.

One former friend recalls that in the early years, at least, "he was like all the other kids, very smart in school, quite active."

Katsiroubas appears to have had an older brother, and at least three large families of cousins.

One relative is quoted in a 2007 news article saying he travelled all the way across London to shop at his favorite Greek pastry shop.

Little is known about Ali Medlej; he appears to have gone through most of his schooling without even a mention in most of his school yearbooks.

But sources tell CBC News that he and Katsiroubas remained friends throughout their high school years together at London South Secondary.

Medlej appears to have graduated in 2007, the same year Katsiroubas seems to have dropped out at the end of Grade 11.

A former friend says that at some point in those teenage years, Katsiroubas converted to Islam.

One former school acquaintance who ran into Katsiroubas in 2009 recalls "it was really hard to relate to him at that point. He wasn’t the same. He had other interests — kept saying let's go to the mosque.

"It wasn’t that he wanted to take me there. It was that he wanted to go there and he thought a couple of hours spent with me [catching up] was probably a waste of time."

Shortly after they left high school, Medlej and Katsiroubas and another friend travelled out West, did some odd jobs, and finally returned to London within six months.

Medlej apparently got married sometime after 2009.

The next time most of their former school friends heard about Medlej and Katsiroubas again was about two months ago after the attack on the Algerian gas plant.

CSIS agents were back asking questions again.

The agents never mentioned the two were dead, much less that they had participated in one of the most heinous attacks in recent years.

Investigators say many of the innocent gas plant workers who died in the attack were burned alive.