A 22-year-old Canadian-born Muslim convert who left Calgary for Syria in November 2012 has been killed by Free Syrian Army (FSA) forces during rebel infighting, CBC News has confirmed.

Mustafa al-Gharib​, born in Nova Scotia as Damian Clairmont, was an Acadian who spent his first years in Wedgeport, N.S.

According to sources in Syria and Canada, al-Gharib was injured in battle and subsequently captured and killed by an unknown faction of the FSA in the city of Aleppo.

Al-Gharib was reportedly fighting with Jabhat al-Nusra, an al-Qaeda-affiliated rebel group consisting of largely foreign extremists.

Jabhat al-Nusra was designated a terrorist group by the Canadian government in November 2013.

The first public indication of al-Gharib's death came on social media on Tuesday night, when a Twitter account claiming to be run by a rebel fighter who knew al-Gharib personally tweeted a martyrdom notice. The notice uses the name Abu Talha al-Canadi, another of al-Gharib’s monikers.

Troubled teen to extremist

Al-Gharib converted to Islam following a two-year period of personal anguish in his teens that included him dropping out of high school, a diagnosis for bipolar disorder and a suicide attempt at age 17.

“He had some trouble as a teenager. When he converted to Islam, initially his family thought that this would be the thing that would calm him down. And eventually it did,” said CBC News senior correspondent Adrienne Arsenault, who interviewed al-Gharib’s mother last year.

“He did seem to find some peace. And then he changed.”

Jabhat al-Nusra publicly announced its creation in Jan. 2012 and reportedly consists mostly of Sunni Islamists who seek to overthrow the regime of Syrian president Bashar al-Assad. The group operates largely in and around the city of Aleppo and Syria's north. (Molhem Barakat/REUTERS)

Shortly before his 20th birthday, he moved into a boarding house and, according to his mother, became secretive, isolated and “very angry, very political,” Arsenault said.

In November 2012, al-Gharib informed his mother that he was leaving Canada for Egypt, where he would study to become an imam.

Several weeks after his departure, however, CSIS contacted his mother and told her that al-Gharib had in fact not gone to Egypt, but rather flew to Istanbul and made his way into Syria shortly after, where he joined up with an extremist group.

CSIS told his mother that it had been watching al-Gharib before he left, but it was unable to stop him from leaving.

Al-Gharib’s contact with his mother became increasingly infrequent as time passed, and his text messages “became increasingly angry about Canada, increasingly determined to be there fighting,” said Arsenault, based on conversations with his mother.

Rebels fighting rebels

Infighting among rebel factions in Syria's north has intensified in recent weeks, with the hardline extremist organization Islamic State of Iraq and the Levant (ISIL) reportedly alienating other rebel groups with brutal tactics and enforcement of Sharia law.

While ISIL and Jabhat al-Nusra — both recognized branches of al-Qaeda — originally fought alongside secular and moderate rebel groups against the forces of President Bashar al-Assad, ISIL is now focusing its attention on taking control of cities held by other rebel forces.

The push has pitted the ISIL against the FSA, as well as its former allies Jabhat al-Nusra.

U.K.-based Syrian Observatory for Human Rights estimates that nearly 500 people, 85 of them civilians, have died in rebel-on-rebel clashes this month.

Unknown number of Canadians in Syria

Although it remains unclear how many Canadians are fighting in the Syrian civil war, estimates range from a dozen to more than 100, said Arsenault.

Bilal Abdul Kareem, an American filmmaker who lived among Islamist fighters for more than a year, told CBC News in September that he had encountered between 20 and 30 Canadians fighting with various rebel groups.

Ali Mohamed Dirie, a member of the so-called Toronto 18 who left Canada on a fake passport in 2012, was killed last year while fighting with Islamist rebels, according to sources close to his family.

Similarly, a Canadian-born convert known as Abu Muslim, who appeared in a Channel 4 documentary in June 2013 travelling with a rebel group called Katiba al-Muhajireen, reportedly took part in a raid on an airport in Syria’s north.

Reports surfaced from a jihadi group on social media that a Canadian was killed in the attack, but it remains unclear if it was Abu Muslim or another fighter.

CBC News currently has anecdotal evidence of several other Canadian deaths in Syria, but the Department of Foreign Affairs has not provided official numbers.

"We are aware of reports that Canadians have been killed in Syria. We are following the situation closely," said a spokesperson for the Department of Foreign Affairs in an email statement on Wednesday.

"Since April 24, 2011, Canada has advised against all travel to Syria due to the deteriorating situation."

On Wednesday, Foreign Affairs Minister John Baird told reporters that the news of al-Gharib's death didn't "come as a surprise" and that "there is probably more than one Canadian that is fighting with the opposition."

Canada does not have exit controls for people leaving for Syria, and therefore it is very difficult to keep track of potential extremists abroad, said Arsenault.