A 16-year-old friend of the gunman who killed nine people in the German city of Munich on Friday has been arrested by police, Sky News reported late Sunday.
Investigators say the Afghan teenager is suspected of knowing about David Ali Sonboly's planned attack and failing to report it.
Munich police added that the friend may have played a role in a Facebook post which invited people to a meeting place in the city center, according to Sky News.
German media reported the arrested boy had gone to the police himself in the aftermath of the shooting.
Earlier on Sunday it was reported that Sonboly planned his attack for an entire year.
According to the Bavarian State Crime Office, Ali David Sonboly had illegally purchased the Glock pistol he used in the attack online. He also left a manifesto of some description on his computer - though its details have not been released.
He is believed to have drawn crowds of victims to the site by offering free McDonald's mealsat the Olympia shopping center, via a hacked Facebook page.
Though his crime was premeditated, at this point investigators believe the specific victims themselves were shot at random, and note that none of them were Sonboly's classmates. Seven of those killed were teenagers.
Sonboly's motives are not yet known, but he was reportedly obsessed with previous deadly mass-shootings by teenage attackers and had a history of mental health problems. He was also also an avid player of first-person shooter computer games.
Bavarian prosecutor Thomas Steinkraus-Koch confirmed Sonboly feared contact with others and had previously been treated for mental health problems at a psychiatric inpatient in 2015.
Police have been probing claims that the killer felt bullied by his peers and that he may have been inspired by Norwegian mass murderer Anders Breivik, who killed 77 people exactly five years before the Munich shootings.