First thing everyone's asking:
"Why is this man able to procure weapons?"
Complete opposite reaction to the US, where it would be:
"We need more weapons to protect ourselves from people like this."
In Belgium it is a statistical anomaly. In the US it is everyday life.
It is everyday life almost everywhere. Except for few West. European countries.
As to your last part, I think you do know that the States do live for this, selling weapons. So there's no point really questioning how/when did he or they procure weapons like in Belgium, Switzerland, Netherland or Austria do. They're completely different structures and irrelevant things since ages.