A police officer stands guard behind Belgian and Israeli flags at the Jewish Museum in Brussels on June 1, 2014.
Europe can expect further "small-scale attacks" like the fatal shooting at the Jewish Museum in Brussels, the EU's anti-terror chief Gilles de Kerchove said on Monday.
"I don't expect another 9/11," he said in a video statement. "I don't expect a major sophisticated attack."
"I expect more small-scale attacks like the one in Brussels," he added referring to the May 24 attack that left three dead and a young man fighting for his life.
A 29-year-old Frenchman, Mehdi Nemmouche, who spent more than a year fighting in Syria, is being held in custody on suspicion of the attack after being detained Friday in Marseille.
De Kerchove said more than 2,000 Europeans had left for Syria or were on their way there, where "they will get military training, become much more radicalised, and have a network across Europe."
Though "few will mount an attack", the Brussels incident was "very, very worrying" for Europe's future security, he said.
De Kerchove said that though it remained to be seen whether or not Nemmouche acted on his own inspiration, "I don't think he is a lone wolf."
The attack however showed the need for European Union nations to coordinate against the threat of jihadist fighters returning home from the Syrian front.