Reports state that the teenager who shot dead 9 people at a Munich shopping mall spent a year planning the rampage but selected his victims at random.
Police has arrested a 16-year-old Afghan friend of the gunman in connection with the attack.
Details are emerging of attacker David Ali Sonboly as a depressed 18-year-old, obsessed with mass killings, and had long struggled with his mental health.
Police said that incident did not bear the hallmarks of a "terrorist attack" and was more likely to have been a crime of passion.
However a string of attacks in recent months claimed by the Islamic State group has left Europe on edge.