"How is it possible that they have developed so much capacity, while the rise of IS stayed unnoticed?”"
If this Dutch MEP wants to know about the rise of IS/SIS/ISIL he can just look at Wikipedia.
I mean, it's one thing to criticize the US and UK governments for hacking into stuff, but to then say they didn't know about IS seems a bit weird. Of course they knew. But what, exactly, could they have done? Military intervention in Syria? Tried that, didn't get through Parliament or public opinion. Increasing troops in Iraq? Not going to be popular when people wanted them out in the first place. Etc.