passport as a refuge, that is going to spur more fears about what the threat is and how to contain it. to be very clear, investigators do believe more were involved, yes? it's just about isolating where and who? >> reporter: absolutely but it is often seen this is a crucial link in that terror pipeline between europe and syria and authorities here believe the man they are looking for, that is the brother of than isis fighter currently in syria, so this is part of the broader concerns and the fear that is threatening to engulf europe about the ability of people to move not just in syria to fight but to move back. at the end of last year, belgian authorities said at least a few hundred belgiumians who had gone to fight in syria and almost a hundred they believe had come back and they were trying. the crucial word is trying to keep tabs on them but not all of them have been on the authority's radar.