Seven of his victims were teenagers, five of them under 16.
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“There is a suspicion that the 16-year-old is a possible tacit accomplice to [Friday’s] attack”, police said in a statement Sunday, reported by broadcaster Deutsche Welle.
Police have raised the number of people injured in Friday’s rampage from two dozen to 35.
Bavarian investigator Robert Heimberger said the shooter visited the site of a previous school shooting in the German town of Winnenden and took photographs past year, then set about planning Friday’s attack, according to Associated Press.
Investigators also concluded that he bought his weapon – a reactivated Glock 17 pistol – on the darknet.
He received inpatient psychiatric treatment in 2015 for two months.
The teenage gunman who shot nine people dead in Munich had been planning his attack for a year, according to German authorities.
Sonboly carried out his sickening attack on the fifth anniversary of Norway terrorist Anders Breivik’s massacre in 2011, where 77 people were killed.
Investigators say the Afghan teenager is suspected of knowing about David Ali Sonboly’s planned attack and failing to report it.
Residents in the capital, Pristina, said they were horrified by the shooting.
The attacker set up a fake Facebook account in May, stealing the profile of a real user and sending out invitations to lure people to the fast-food restaurant.
“The suspect had fears of contact with others” and also depression.
Munich’s police chief has urged the media to respect the privacy of those affected by the attack on Monday, when schools reopen.
Mr Heimberger said German-Iranian Sonboly’s parents were still in shock and detectives had not yet had a chance to interview them.
“He completely occupied himself with this act of rampage”, Heimberger said.
Since the 2009 attack at the school, there had not been a mass shooting of four or more people killed in a single event in the country.
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Police had said Saturday that the attacker was a mentally troubled individual who had extensively researched rampage killings, and had no apparent links to terror groups and no political motive.
