Here's an important issue that has not been mentioned much but which you have to hope the relevant authorities are looking seriously at: where did they get their weapons from? Even in a state as dysfunctional as Belgium you can't just go out and buy an AK-47.
Honestly it's not that hard to get access to weapon in most European countries, and it's even easier if you have connection to criminals, which you often find among Islamist groups. You can't do a lot about it, except throwing the book after the people who sold them to them. After the attack in Copenhagen the Danish judicial system began to go after the people who got the weapon to El-Hussein, and they have decided to use the terror law against them.