Munich (Germany): The German-Iranian teenager who shot dead nine people in Munich on Friday started planning the attack one year ago, a German official said on Sunday. David Ali Sonboly, 18, began the planning after visiting the scene of a 2009 school shooting in the German town of Winnenden, a Bavarian official said, Xinhua news agency reported. According to German authorities, the teenager did extensive research on mass shootings before carrying out the attack. He stored a copy of the manifesto of Norway attacker Anders Behring Breivik in his computer, according to Bavarian Interior Minister Joachim Herrmann. Breivik killed 77 people in Norway in 2011, many of them attendees at a youth camp. The shooter regularly played a video game "Ego-Shooter", which features mass killings, according to a police statement. German prosecutors said the shooter acquired the semi-automatic pistol he used in the killings on the dark web, which can only be accessed via special software. Police and prosecutors confirmed that the shooter, who received medical treatment for a long time, stayed in a closed psychiatric ward from July to September 2015, while the last treatment with a doctor could be traced to June 2016. According to police, 58 bullet casings were found at the shooting scene and 57 of them were from the gunman's weapon, while one was from a police weapon. Based on the findings, the police and prosecutors announced there was no second suspect. Among the nine victims, five were men and four women. Another 35 people were injured, 10 of whom seriously. David Ali Sonboly killed himself after the attack.