(CNN)The gunman who killed nine people in a rampage in Munich on Friday was obsessed with mass shootings and appeared to have planned the attack for a year, officials said.
"He appears to have planned this act since last summer," Robert Heimberger, president of the Bavarian state criminal police office, said at a press conference Sunday.
"He completely occupied himself with this act of rampage."
Police have not named the attacker, but said he was an 18-year-old with dual German and Iranian nationality who was born and raised in Munich. Neighbors told CNN Saturday that the teen who lived in an apartment searched by police was Ali Sonboly, a name reported by German media outlets.
Police had said Saturday that theattacker was a mentally troubled individual who had extensively researched rampage killings, and had no apparent links to terror groups and no political motive.
On Sunday, investigators revealed he had left behind a long written statement on his computer, which was still being analyzed. They also said they had found photographs on his camera showing he had also visited the German town of Winnenden, the site of a deadly 2009 school shooting.
Earlier, officials said they had found in the gunman's belongings numerous documents on mass killings, including a book entitled "Rampage in My Mind -- Why Students Kill."
Officials also believe there likely was significance in the timing of the attack, which came five years to the day since Anders Behring Breivik killed 77 people in Norway in 2011, many of them attendees at a youth camp.