"He had been preparing (the shooting) for a year," Bavarian police chief Robert Heimberger told a news conference.
Chief prosecutor Thomas Steinkraus-Koch added that he did not specifically choose his victims.
"It is not the case that he deliberately selected" the people who he shot, he said.
German-Iranian shooter kills at least nine in Munich attack
Europe reacted in shock to the third attack on the continent in just over a week, after David Ali Sonboly went on a shooting spree at a shopping centre on Friday before turning the gun on himself.
Officials said Saturday that Sonboly, a German-Iranian student, had a history of mental illness.
Investigators said they saw an "obvious link" between the killings and white supremacist Anders Breivik's massacre of 77 people in Norway exactly five years earlier.
Chancellor Angela Merkel said Munich had suffered a "night of horror".
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