isis about a quarter have come back. but we know at least two probably three of those involved last friday had gone to syria, come back undetected. even their families had no idea that they had come back. the belgian justice minister said, we hadn't a clue that one of these attackers was back on belgian soil. he made the point they don't have the resources to track these people. every single fighter that comes back from syria or iraq is estimated at 25 officers, analysts are acquired to track their moverments. this is the scale of the charge. and the europeans are really scrambling to catch up with the flow of both outward and backwards. on top of which you have the silos of intelligence, intelligence agencies very zealous of their sources, information. they're not sharing enough information. it's a huge challenge, chris. >> tim lister, stefan due vees, thank you very much. you would think the authorities would be celebratory today.