The search for answers began immediately after the tragedy. A picture of a lonely, frustrated loner began to emerge - but little to suggest that Tim Kretschmer was planning such a terrible crime. The blame was initially pointed at violent computer games, with calls for a ban. But the focus quickly switched to legally held firearms, when it emerged the teenager had stolen a gun from his parents' bedroom.The case has led to a tightening of Germany's gun laws. German schools have also introduced measures to pinpoint signs of aggression among pupils, and prevent similar crimes happening again.