Reports have stated that a former Guantánamo prisoner has died fighting alongside rebels in Syria.
A video purporting to show the body of one of the captives released during the Bush administration has been released on YouTube.
Sheik Abu Ahmad al Muhajir, eulogised as Mohammed al Alami, joined a Syrian insurgency to topple Bashar Assad, The Miami Herald reported.
Now, experts have said it was only a matter of time before foreign fighters became aligned with rebel groups in Syria.
"Obviously there've been cases of people [Guantánamo detainees] who've gone back to continue the fight," Aaron Zelin, editor of the jihadology blog from the Washington Institute for Near East Policy, said speaking to the paper.
"Syria is probably the biggest jihad since the jihad in Afghanistan in the 1980s," Zelin said, and for "foreigners seen as more of a clean jihad" than attacking U.S. troops and targets," he said.
"It's not surprising. But it is interesting," he added.
The news came as figures were released this month revealing that nearly half of the 100,000 fighters battling President Bashar al-Assad's forces in Syria are aligned to hardline Islamist or jihadist groups.
Some are even linked to al-Qaeda, according to the new analysis of the country's conflict.
Around 10,000 jihadists, who would include foreign fighters, and another 30,000 to 35,000 hardline Islamists are battling the Syrian government, a study from defence consultancy IHS Jane's reported in The Daily Telegraph claims.
After more than two years of fighting in the country, the 100,000 rebels have reportedly been fragmented into as many as 1,000 bands.
Fears that the rebellion against the Assad regime is being increasingly dominated by extremists has fuelled concerns in the West over supplying weaponry that will fall into hostile hands.
Syria has turned over "material evidence" to Russia which shows that a chemical weapons attack in Damascus was carried out by the rebels, a Moscow diplomat has claimed.
Sergei Ryabkov, Russia's deputy foreign minister also accused the report carried out by UN inspectors of being "distorted" and "one-sided", according to the BBC.Suggest a correction