Suspects in Algeria attack had been on Mounties" radar The two Canadians who died in January's terrorist strike in Algeria were under RCMP investigation for two years, suggesting law-enforcement authorities have long been concerned about their potential crimes, sources say. While Ali Medlej and Xristos Katsiroubas are now dead, the Mounties are still seeking more information on anyone in Canada who might have assisted the pair's terrorist endeavours or participated in their radicalization in recent years. "The reality is that we need the public's help," said RCMP Superintendent Marc Richer at a news conference. The RCMP confirmed the identity of the two individuals from London, Ont., for the first time on Thursday, while Foreign Affairs confirmed that a third Canadian was "detained abroad." Aaron Yoon is in custody in Mauritania, said a staffer at the West African country's mission at the United Nations. Mr. Yoon from London, Ont., is a high-school friend of Mr. Medlej and Mr. Katsiroubas, who were among the 29 terrorists who died in an attack against a gas plant in Algeria. Asked specifically if a Canadian citizen was held in Mauritania, a Foreign Affairs official would only say that "we are aware of a Canadian who has been detained abroad." Supt. Richer said the RCMP's call for the public's help, and the confirmation of the identity of the deceased pair in the middle of the ongoing investigation, is an exceptional situation for the national police force. He said the RCMP now wants to find out more about the deceased's acquaintances, influences and enablers, acknowledging the "overwhelming interest" in the attack across the country. "If people are sitting on nuggets of information that could be helpful to us, they can share them with us," he said. Supt. Richer added the investigation into the pair goes back "quite some time," although he refused to lay out key details of the probe for the public at this point in time. "It is not something that we [set up] last month," he said, while another source added the matter goes back more than two years. The revelation suggests the Canadian Security Intelligence Service, which was on the case years earlier, could have given the Mounties a heads up about the group through the formal legal mechanisms meant to neutralize the most serious terrorist threats to Canada. Usually, the two federal counterterrorism agencies work to very different investigative standards, with CSIS being given broader latitude to collect information, and police detectives facing higher legal thresholds for investigations, given how they alone reserve the right to arrest suspects. This dynamic means that many RCMP investigations and CSIS probes are conducted in parallel. However, 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 CSIS disclosure letters provide tips and leads to the Mounties, while CSIS advisory letters provide the RCMP with information that is intended to support police searches, wiretaps and, eventually, criminal charges. Given the Londoners were on CSIS and then the RCMP's radar screens, that means they would have been considered to be a dangerous and elevating threat. Mr. Yoon, who grew up as a Catholic, and Mr. Katsiroubas, once Greek Orthodox, converted to Islam. A friend who knew Mr. Katsiroubas and Mr. Medlej said he had been questioned about them by CSIS in 2007, when the two were living in Edmonton. On Wednesday, Mr. Yoon's brother told The Globe and Mail that they believe he had travelled to Mauritania by way of Morocco but was not detained. He said Mr. Yoon, who converted to Islam three or four years ago, had gone to Morocco to meet a friend and then proceeded to Mauritania. The brother said Mr. Yoon last spoke to his family on Sunday. According to a former co-worker, Mr. Yoon had changed in recent years. In 2008, he was a "fun-loving" teenager who worked part-time at a local restaurant and hung out with friends from work. When the co-worker met him again in 2011, he was praying several times a day and refusing to talk to his female supervisors. 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. With reports from Ann Hui, Kathryn Blaze Carlson and Colin Freeze