French police have arrested 29-year-old Mehdi Nemmouche today in Marseille on suspicion of being the attacker in last weekend’s shootings at the Jewish museum in Brussels.

Nemmouche, a French citizen, was arrested with an assault rifle, and reportedly confessed to the Brussels attack. The big eye-opener, however, is that he had recently returned from Syria.

Nemmouche is one of thousands of Westerners who went to Syria to join jihadist rebel factions, and reportedly fought with al-Qaeda in Iraq (AQI) while in Syria. France estimates 700 of its citizens alone have fought in Syria.

The attacker’s back-story puts a face to growing fears among European security officials that jihad-enthusiasts who go to fight in Syria will return with an eye on conducting high-profile attacks within the EU.