ISIS doesn`t work like we think in my opinion. They spread their people all over Europe, they have some "contact-agents" probably but mostly...they are "normal people" or seem to be normal people hidden or frozen till the attack. In my opinion they work on their own very much and have a free choice of methods and weapons. Since the first "truck" attack some of them just repeat it. But they are not typicull "agents", they are waiting for their moment and there is a lot of them, of this i`m sure, and probably very dangerous muslims are in EACH AND EVERY European country.
Another attack is just a question of time, like in DM song. Solution? There is no..apart from...when strong-right parties will really start to win elections and the situation will change.
There will also come a time when 1 attack will cross the border...cuz some kid will die or whatever. There is always a border, always and majority always reacts event if we have natural instincts lowered in 21st century. Sadly....from reasons hidden in our "DNA" majority will react and will start to defend from minority, this may be a goal for ISIS, to start sth bigger between 2 cultures and religions