The arrests and recent convictions of two men from Russia's predominantly Sunni Muslim Republic of Tatarstan for fighting in Syria, possibly with the Islamic State (IS) militant group, has raised concerns that extremism could be on the rise in the region.

Fears that Russia's Muslim population is at risk of radicalization from groups like Islamic State have soared in recent months, amid daily reports in the Russian media of the IS group's brutality in Syria and Iraq.

Most reports of Russian nationals returning home after fighting with IS and other factions in Syria have been in the North Caucasus. However Russia's Tatarstan region found itself in the spotlight in recent weeks after two ethnic Tatars were convicted and sentenced to prison terms for fighting in Syria.

In December, a court in Kazan sentenced 27-year-old Raif Mustafin to three years and 10 months in prison for fighting in a militant group in Syria. Last month, a 25-year-old resident of Naberezhnye Chelny in Tatarstan, Rishat Idrisov, was sentenced to one year in prison, also for fighting with a militant group in Syria.

The indictment in Mustafin's case said that he was a member of a "Tatar jamaat [fighting group]" in Syria.

That the convictions of Mustafin and Idrisov provoked or exacerbated fears of radicalization among Russia's ethnic Tatars was evident in the media response to the two court cases.

The local Tatarstan edition of the Russian newspaper Moskovskiy Komsomolets (MK) claimed that Syria has become "a new training camp for Wahhabis from Tatarstan." (Wahhabi is a term used in Russia and elsewhere in the former Soviet Union for followers of radical Islam.)

"Now they are returning home in order to build an Islamic state in the Volga Region," MK continued, repeating fears that returnees from the conflict in Syria and Iraq could form radical groups in their home regions.

MK blamed the phenomenon of Tatars fighting in Syria on what it said was a rise in radicalization in Tatarstan. This, MK claimed, was the result of the "religious renaissance" that occurred as a result of "outside influence" following the collapse of the Soviet Union in the early 1990s. According to MK, the main problem was Tatarstan's desire for autonomy and its pursuit of an "active foreign policy" via which its leaders "held willing meetings at the highest political level in Muslim countries."

As a result, "nontraditional streams of foreign forms of Islam of a radical bent" were spread in Tatarstan, and Islamists from the region even went to fight in Chechnya, MK added.

According to MK, since the Syrian crisis began in 2011, would-be militants from Tatarstan have crossed into Syria via Turkey, posing as "ordinary Russian tourists," a claim that is likely based not on specific evidence regarding Tatars but on the fact that most of the militants fighting in Syria traveled to that country via Turkey.

Beyond the fears, as expressed by MK, of blowback from Syria returnees in the form of domestic terrorism and the rise of "Wahhabi groups" in Russia, what evidence exists of a Tatar presence in Syria?

There are no official figures for Tatar militants -- either from Tatarstan in Russia or Crimea in Ukraine -- fighting in Syria, although a figure of 500 Crimean Tatars has been posited.

That estimate is heavily politicized, however. It was postulated in November by Ruslan Saitvaliyev, the head of the recently revived pro-Moscow Crimean Muftiate of Taurida, which was formed after Russia's annexation of Crimea last March. His estimate that as many as 500 Crimean Tatars were fighting with the Islamic State group in Syria was part of a warning that Russian-speaking Islamic State militants pose a serious threat to Crimean stability.

A recent infographic published by The Washington Post estimated that 50 individuals from Ukraine have gone to fight in Syria, although there is no specific evidence given to explain or support this figure.

No estimate for the number of Tatars from Tatarstan has been suggested.

The main sources of evidence for the presence of ethnic Tatars from both Crimea and Tatarstan in Syria are testimony given at Mustafin's trial in Kazan, and social media generated by Russian-speaking militants in Syria.

These sources indicate that there are likely two different Tatar groups in Syria, one for Crimean Tatars and one for Tatars from the Republic of Tatarstan.

The Chechen-led faction Jaish al-Muhajireen wal-Ansar (JMA) has claimed to have a Tatar faction within its ranks, led by a Crimean Tatar named Abdul Karim Krymsky.

Little is known about the background of Krymsky, but his position as JMA's military commander suggests he has some fighting experience. Krymsky first appeared in a video distributed on the Internet in May 2013. The video shows an address given by Krymsky, who is lying wounded in a hospital bed and who is described as the leader of a Tatar jamaat. The video was distributed by the FiSyria media group, which is close to Umar al-Shishani, who was the leader of JMA at the time the video was made.

Although Krymsky is fighting in Syria, he has clearly maintained strong emotional ties to Crimea and in a video address in May openly called on Crimean Tatars to "recommit themselves to Islam" and "return to jihad" or face another mass deportation.

It is notable that Krymsky did not refer to Tatars outside of Crimea; neither did he call for Crimean Tatars to fight in Syria but urged them to follow Islam or face national crisis.

While there has been hardly any other open source evidence of individual Crimean Tatars fighting in Syria, there have been reports that a militant from Nizhnegorsk in Crimea carried out a suicide bombing attack for JMA in April 2013, while the faction was still under the leadership of Shishani, now the IS group's military commander in Syria.

According to the FiSyria media group, the militant was named Ramazan or Abu Khalid, and married a local Syrian woman a week before he carried out his suicide truck bombing against Syrian government forces at the Al-Kindi Hospital in Aleppo Province. Photographs taken before the attack show Abu Khalid with Shishani.

By piecing together evidence, it is likely that in addition to the Crimean Tatar-led faction in the Aleppo-based JMA, there is another, separate Tatar group based in Latakia Province in Syria.

Social media reports from June 2014 testify that a Tatar from Russia's Orenburg district fought and died in the Syrian town of Kassab in Latakia Province.

The militant, named as Abu Jabbar, is described as a veteran militant who had links to extremist groups in Tatarstan.

The report of his death, which was published on the Russian social networking site VKontakte, said that Abu Jabbar was a native of Orenburg Province but lived mainly in Tatarstan.

The militant "started his jihad from Tatarstan, where he is on the international wanted list, but by the will of Allah he slipped away from the hands of the Kuffar [infidels] and came to Syria...He had a wife in Syria and six kids; the youngest was 2 months old yesterday," according to the post.

The report of Abu Jabbar's death in Latakia Province is notable because reports of the trial of Kazan resident Mustafin note that in January 2014, he had been based with the so-called "Tatar jamaat" in a "population center in Latakia."

The reports also say that the "Tatar jamaat" is part of JMA; however, at the time that Mustafin was with the group, and in June 2014 when Abu Jabbar was killed, JMA had no presence in Latakia, a fact that supports the hypothesis that there are two distinct Tatar groups in Syria.

-- Joanna Paraszczuk