Munich police have said they were questioning a friend of the shooter who killed nine people in a McDonalds on Friday night over allegations he may have known about his plans.
The unnamed 16-year-old reportedly handed himself into police for questions on Sunday evening following Ali David Sonboly's deadly rampage.
Police have described Sonboly, who killed himself after carrying out the massacre, as a withdrawn and depressed loner who became obsessed with violent video games and also mass shooters.
He bought a Glock pistol illegally online with more than 300 bullets and reportedly hacked into a Facebook account to lure more people to the McDonalds in the hope of getting a free meal.
German authorities said the teenager could have been planning the attack for a year.
Robert Heimberger, head of Bavaria's criminal police, said Sonboly had visited the site of a 2009 school shooting in nearby Winneden last year to take photographs, and authorities found a book about mass shootings in his room.
Munich police chief Hubertus Andrae said: "Documents on shooting sprees were found, so the perpetrator obviously researched this subject intensively" but "based on searches" he had no connection to Isis.
In a post on a chatroom that has since been deleted, a classmate said: "I know this f****** guy, his name is Ali Sonboly.
"He was in my class back than (sic). We always mobbed him in school, and he always told us that he would kill us."
In a video posted online following the shooting, a gunman presumed to be Sonboly is seen arguing with another man and claiming he had been bullied for years prior to the attack.
He said: "Because of you I was bullied for seven years ... and now I have to buy a gun to shoot you".
Sonboly was reportedly inspired by Anders Behring Breivik, who killed 77 people in a bombing in Oslo and a mass shooting at summer camp on the island of Utoya.
According to German newspaper, Bild, he had a picture of Breivik on his Whatsapp profile picture.
The Munich shooting happened on the five year anniversary of the Norwegian's rampage.
