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By Anne-Sophie Herrmann and Matthew Grimson

MUNICH — The gunman who killed nine people at a Munich mall on Friday spent at least a year planning the attack and had visited the site of a 2009 school massacre, officials said Sunday.

Police officers stand behind a wreath near the shopping mall where a shooting took place.Jens Meyer / AP

The 18-year-old shooter, identified as David S. by local media, had also received two months of psychiatric treatment last year, according to Thomas Steinkraus-Koch, a spokesman for the Munich prosecutor's office.

Bavarian official Robert Heimberger said evidence found at the home of the shooter, who turned the gun on himself after the attack, showed he was an avid player of first-person shooter games.

Related: Munich Witness Describes Comforting Dying Boy

Investigators found a manifesto at the shooter's house and a digital camera that showed he'd visited the site of a high school massacre in Winnenden, southern Germany, where 15 people were killed before the 17-year-old gunman committed suicide.

Heimberger said the shooter likely obtained his weapon, a 9mm pistol, illegally via the dark web. Investigators did not know where he purchased the ammunition, he said.

"At the crime scene 58 shells were found," he said. "From those, 57 were from the weapon of the attacker. The 58th one [belonged] to the police. Because of that it is a matter of exclusively one individual offender."

Steinkraus-Koch said there is still no evidence of any political motivation to the crime, nor that the shooter killed specific victims. The shooter had "social phobias," Koch said, which may explain his behavior during the attack.

Officials had previously said it appeared the shooter had hacked the Facebook account of a young woman to lure victims to the attack. However, Heimberger said on Sunday that the account was a fake.

Investigators also walked back on previous statements that the shooter had researched the 2011 massacre in Norway by Anders Breivik. They said no such information had so far been found on his laptop, which they were still investigating.

Heimberger said the parents of the gunman were still in shock and were not able to be interviewed.

Around 3,000 people are expected to attend a prayer vigil Sunday evening.

Andy Eckardt and The Associated Press contributed.