At least one of the two Canadians who took part in a deadly attack on an Algerian gas plant had worked at the facility, CBC News has learned.

Militants took control of the Tigantourine gas plant near Ein Amenas, Algeria, on Jan. 16. After a four-day hostage standoff, Algerian special forces stormed the facility. More than 80 people were killed in the violence.

An extremist group called the Masked Brigade claimed responsibility, saying it was retaliation for Algeria's support of France's miiltary offensive against Islamist insurgents in neighbouring Mali.

Two young men from London, Ont., 22-year-old Xristos Katsiroubas and Ali Medlej, who was about 24 years old, are believed to have died fighting alongside the militants during the refinery attack.

hi-algeria-suspects-4col

Xristos Katsiroubas, left, and Ali Medlej have been identified as the young Canadian militants involved in the attack at the Algerian gas plant. (CBC)

The pair travelled to Morocco in 2011 with Aaron Yoon, another young man from London. All three made their way to Mauritania, where Yoon is currently imprisoned.

A CBC News investigation has learned that Medlej was briefly detained in Mauritania in the late fall of 2011, suspected of plotting an attack. He was briefly questioned, held for 40 days and released without charges. Those who interrogated Medlej found him to have no ill intent and no malice, but they seem to have seriously misread the young Canadian’s objective.

Sources at the jail in Nouakchott, the Mauritanian capital, say that as soon as Medlej was released in early 2012 he went to find work at the gas plant in Algeria, where he would be killed a year later.

It seems his role was to scout the facility. He then travelled to militant training camps in Mali to help draw up the plans of attack, sources say.

That assessment fits with Mauritanian security analyst Isselmou Ould Moustapha’s view of the attack. Targeting such a large piece of infrastructure as the Ein Amenas plant would have required "good information about what is happening there. Where are the doors, where are the windows, where are the people living?" he said.

"They had the information. They had solid complicity."

Canadian sources confirmed that at least one of the Canadians worked at the plant, but it's possible that both had been employed there. Several reports suggest a number of the hostage-takers were familiar with the facility.

Training the attackers

Logistical planning for such an attack would have been intricate, but training the militants to kill may not have been a long process, Moustapha said.

He estimates that, according to his sources, Medlej and Katsiroubas would have been in the northern Mali training camps for between two weeks to two months starting in the fall of 2012.

'Where are the doors, where are the windows, where are the people living? They had the information. They had solid complicity.'—Mauritanian security analyst Isselmou Ould Moustapha

Yoon, the young man who travelled to Mauritania with Katsiroubas and Medlej in 2011, was arrested in December of that year for allegedly having ties to a terrorist group, and remained imprisoned in Nouakchott when his two friends would have departed for Mali.

Yoon, who has always maintained his innocence, received a two-year sentence after authorities suspected him of trying to visit militant training camps. His trial and sentencing happened before the Algeria attack, and before Mauritanian officials understood that Yoon was friends with Medlej and Katsiroubas.

Mauritanian prosecutors have now asked for a retrial for Yoon and several others imprisoned on terrorism-related convictions. A hearing is scheduled for next week. Prosecutors feel the sentences weren’t strict enough and it seems they have new questions. 

A judge could choose to release Yoon, reduce his sentence or order him to serve more time behind bars. Yoon, who has always maintained his innocence, has turned down previous offers of legal help. But a human rights worker dealing with his case said Yoon’s attitude is shifting and he may now realize he needs a good lawyer.

Searching for clues in Mauritania

A team from CBC News spent a week attempting to retrace the steps of the three Canadians after they arrived in Mauritania.

It seems they made curious choices. Instead of staying at one of the well-known, inexpensive backpacker hotels popular with young travellers and students, they opted for a guesthouse in the centre of what police call a criminal hub.

It’s a neighbourhood that extremists are known to frequent, where the tension is palpable. It’s not the sort of place you find in a travel book, suggesting that someone may have guided them there.

The contacts that the trio made and the places they went all suggest someone was on hand to give them advice. Just who that person is counts among the many secrets the desert isn’t quite willing to give up yet.