The teenage gunman who killed nine people in Munich on Friday had been planning his attack for a year, German authorities say.
David Ali Sonboly, 18, who killed himself after the attack, had a Glock pistol that police believes he may have bought on the so-called dark net.
Meanwhile police in Munich have announced the arrest of a 16-year-old Aghan friend of the attacker.
They say he is under investigation for not reporting Sonboly's plans.
A statement on Munich police's Facebook page says: "There is a suspicion that the 16-year-old is a possible tacit accomplice to the attack."
The statement goes on to say that the youth reported to police immediately after the shooting on Friday, and was interviewed as somebody with a connection to the attacker.
But in the course of the interviews, they discovered discrepancies in his statements.
They say they are now investigating him on suspicion of failing to report a planned crime.
The investigations will have to show to what extent he was responsible for a Facebook post inviting people to meet at a cinema complex near the main railway station in Munich.
The attacker himself had put up a Facebook post before his attack, inviting people to come to the fast food restaurant where the shooting began.
'Not specifically targeted'
Seven of the dead in Friday's shooting at the Olympia shopping centre were were teenagers - two Turks, two Germans, a Hungarian, a Greek and a Kosovan.
A further 35 people were injured, but only four of them have bullet wounds - many were hurt while fleeing the scene.
The state government officials told a news conference that the victims of the attack had not been specifically targeted and were not classmates of the gunman.
Also they did not include three youths allegedly involved in bullying Sonboly when he was at school.
Robert Heimberger, head of Bavaria's criminal police, said the gunman had been planning the attack since he paid a visit last year to the town of Winnenden - the scene of a previous school shooting in 2009 - and took photographs.
He said it was likely the Glock pistol - which had been reactivated - was bought on the "dark net" market, an area accessible only with the use of special software. It had been a theatre prop.
Sonboly was said to be a keen player of "first-person shooter" video games.
Mr Heimberger added that the parents of the gunman remained in shock and were not able to be interviewed.
He also said police had not found the manifesto of Norwegian mass killer Anders Behring Breivik when they searched the gunman's room at his parents' flat.
A day earlier, officials had raised the possibility of a link to Breivik, whose own attack was carried out five years earlier to the day.
As to Sonboly's state of mind, a spokesman for the Munich prosecutors' office told the news conference that the gunman had spent two months as an inpatient at a mental care facility in 2015 and was afterwards treated as an outpatient.
"The suspect had fears of contact with others" and also depression, Thomas Steinkraus-Koch said.
However, there was no evidence of any political motivation.
Senior German politicians have called for tighter controls on the sale of guns in the wake of the shooting at the Olympia shopping centre.
Munich's police chief has urged the media to respect the privacy of those affected by the attack on Monday, when schools reopen.