The 18-year-old gunman behind the shooting at the Munich shopping mall on Friday spent at least a year planning the attack, and visited the site of a 2009 school massacre, NBC News reported.
The shooter, identified by local media as David S., also received two months of psychiatric treatment last year, according to a spokesman for the Munich prosecutor’s office.
Evidence found at the shooter’s home showed he was an avid player of first-person games. Investigators also found a manifesto and a digital camera that showed he visited the site of a high school massacre in Winnenden, southern Germany.
Officials say the shooter likely obtained his weapon, a 9mm pistol, illegally through the dark web. It was unclear where he purchased the ammunition.
The shooter killed nine people before shooting himself.