It seems that people planning to do such shootings often "advertise" it on Internet before they act...
Well, in fact, the internet message seems to have been a fake, at least this is what they said on tv today. But they are not sure yet.

But it seems it's always the quiet and shy people, young boys or men who feel they don't get the attention they deserve. In this case, there also seems to have been an underlying mental illness. It is said now that the boy was depressive, but cancelled his treatment. Also, he was an outsider. Someone from his school or neighborhood said "He made it easy for others not to like him". But obviously nobody ever noticed the high level of aggression and rage that must have built up inside of him.

Kirsten