Awso Peshdary, 25, is the only one of the three in custody in Ottawa, the RCMP say. He is facing charges of participating in, conspiring to participate in, and facilitating the activity of a terrorist group.
Also charged is John Maguire, 24, who gained notoriety last year when he appeared in an extremist recruiting video that urged Muslims to launch attacks against Canadians.
Recent reports — including many on social media — indicated Maguire may have been killed in battle, but police say they lack solid proof he's dead.
"I think we follow the evidence," said RCMP Chief Supt. Jennifer Strachan. "In our minds, a tweet is not the kind of evidence we would be looking for to feel confident that Mr. Maguire is deceased."
Maguire, once a hockey-loving teen who attended high school in Ottawa, travelled to Turkey in December 2012, the RCMP say. He is charged with facilitating an activity for a terrorist group.
Khadar Khalib, 23, also believed to be overseas, is charged with leaving Canada to participate in the activities of a terrorist group and counselling a person to participate in terrorism.
Both Maguire and Khalib — whom the RCMP called "active members" of ISIL — are also charged with conspiring to participate in terrorist activity.
The various offences carry maximum penalties of either 10 or 14 years in prison.
The arrests were part of an investigation known as Project Servant, which resulted in charges last month against three other Ottawa men, including twin brothers.
The charges laid Tuesday are a serious reminder of "how we need to remain vigilant against a threat that is persistent, multi-faceted and unpredictable," Strachan told a hastily called news conference.
"This was a complex and extensive operation which involved multiple sensitive investigative techniques."
The accused have posted photographs, videos and written comments on social media that provided evidence to demonstrate their support of extremists, she said.
Peshdary was an associate of Maguire and aspired to travel abroad with him, according to the RCMP. They believe Peshdary stayed in contact with Maguire, and together they entered into a conspiracy to send other Canadians to Syria to join ISIL.
Peshdary and Maguire collaborated with Khalib, who travelled to Syria at the end of March 2014 to join ISIL, the Mounties say.
Strachan noted that Peshdary was arrested as part of another terror probe in 2010 but there was not enough evidence to charge him at that time.
Justice Minister Peter MacKay would not comment on the specifics of the case, but said coming amendments to the Criminal Code will help police in such circumstances.
He rejected suggestions that the latest charges show police already have sufficient powers to combat terrorists and their sympathizers.
"It depends on the circumstances in every case," he said. "Every case is different."
New Democrat MP Jack Harris said police clearly don't need new powers.
"These and other examples show that we already have tools that are effective in arresting and dealing with people who are about to commit terrorism, or are engaged in counselling others to do so," Harris said.
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