An Iranian-German teen who gunned down nine people with a semi-automatic weapon in Munich spent a year planning the attack, and his research of previous shootings included a trip to the scene of a 2009 school massacre in Germany, police and prosecutors said Sunday.

The 18-year-old wrote a manifesto about the crime he planned to commit and an online chat retrieved from his computer indicates he bought the Glock pistol used in the shooting on the dark net, Bavarian police chief Robert Heimberger said.

In 2015, he travelled to scene of a massacre that occurred six years earlier at a secondary school in the southern German town of Winnenden, followed by a shootout at a nearby car dealership. The teen took photos there, which were later retrieved from his camera.

The son of Iranian asylum seekers spent two months in 2015 in a psychiatric hospital receiving treatment for "social phobias" and anxiety and continued treatment thereafter, prosecutor Thomas Steinkraus-Koch said, adding that anti-depressants had been found in his room.

The prosecutor said that the teen regularly played Counter-Strike, a violent first-person shooter video game which he said is played by most perpetrators of mass killings.

The shooter did extensive research on the far-right terrorist Anders Behring Breivik, whose rampage in Norway five years ago left 77 people dead, officials said.

A search of the suspect's home also revealed literature including a German translation of the book, Why Kids Kill: Inside the Minds of School Shooters, by US psychologist Peter Langman.

The fact that six of nine victims had a non-German heritage - including Turkish, Hungarian, Greek and Kosovar - was a complete coincidence, Steinkraus-Koch said, adding that there was no political motivation behind the crime.

Witness accounts and smartphone footage of the attack had indicated the suspect may have harboured resentment towards foreigners even though he was the German-born son of Iranian immigrants.

The prosecutor confirmed that the shooter had been a victim of bullying on several occasions in 2012, but that there were no fellow students among the victims.

Officials said the shooter had tried to lure people to the McDonald's restaurant where many were shot, by offering free meals via a fake Facebook account he created in May.

Officials said that the teen's parents were in no state to be interviewed by police, but that the father had been the first to identify the shooter after recognizing him in video footage circulated shortly after the attack.

Police raised the number of people injured in Friday's rampage from two dozen to 35. The toll includes 31 hurt when panic broke out at the shopping centre and other locations in the city centre. Only four suffered gunshot wounds.

The debate in Germany in the aftermath of the shooting turned to gun control on Sunday, with two members of Chancellor Angela Merkel's cabinet calling for the existing legislation to be reexamined.

"We must continue to do everything in our power to limit access to deadly weapons," Sigmar Gabriel, Economics Minister and Chancellor Angela Merkel's deputy, told the Funke media group on Sunday.

Interior Minister Thomas de Maiziere said that once it had been determined how the teenager had acquired the pistol, it would have to be "carefully examined whether and to what extent there is a need for legislative action."

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