Belgium terror attacks

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I can´t expresss how hanger I´m because of this, many peopple mybe don´t understand, but we are in war. But there is something I don´t know how to explain, every time there is an terrorist attack, there is a huge police search,a lot of investigations, and than they arrested a lot of suspects. So my question is: Why only after the attacks? In the news we listen that the poilice was already investigating some people, so why they heve to wait until an attack hapens? this is just my toughts.