"The latest estimate is that 110 Swedes were confirmed to have travelled since 2012 to join terrorist movements, such as Islamic State, in Syria … Between 25 and 30 have died in combat. Around 40 are confirmed to have come back to Sweden," Kassman said.
Kassman confirmed that the intelligence agency was looking into the circumstances of their travels, but refused to comment on the cases. He stressed however this was an increasingly serious problem that must be tackled with caution.
"It is a disturbing situation that we will be dealing with for a long time," the intelligence agency chief said.
According to Kassman, Syria is becoming the most popular destination for Swedish-born fighters of Muslim descent. Previously, they travelled to fight in wars in Afghanistan and Somalia.
Syria became the flashpoint for the region's jihadist insurgency in 2012 when the Islamic State (IS) terrorist group first reared its head in the country to target government troops of President Bashar Assad. The movement gained steam this summer, spilling over into neighboring Iraq and seizing large swathes of land.
In June, the group declared a caliphate in the territories it occupied and started rallying young Muslims to its cause with the help of social media. According to data obtained by Belgium security services, some 4,000 to 5,000 European teenagers are fighting for the IS in Syria.
Research by the International Centre for the Study of Radicalization and Political Violence in London revealed that one in five foreign fighters in Syria were from western Europe. Most fighters hail from France, followed by Britain, Germany, Belgium and the Netherlands, though Sweden, Denmark, Norway and Austria have also reported a surge in radicalization.