The suspects belong to two separate groups that had links between them, the federal prosecutor’s office said.

Members of one group, based in western Flanders, were allegedly active in Syria and probably participated in combat there, prosecutors said.

One group member returned home to Belgium for medical care after being wounded while fighting with an extremist Muslim faction, authorities said. Some of those arrested allegedly had plans to return to Syria.

The other group, in the Leuven region, had been under investigation since January for possible involvement in preparing an attack, the prosecutor’s office said.

Authorities also performed simultaneous searches in the Belgian cities of Ostende, Bredene, Antwerp, Jabekke, Leuven and Namur, the prosecutor’s office said.