Police say Sonboly, who committed suicide after killing nine people on Friday, was an 18-year-old German-Iranian student who had battled depression and was not previously known to detectives.
He had 300 bullets in his rucksack when he began gunning people down at the Olympia shopping centre just minutes away from the flat that he shared with his family, according to authorities.
Officials suggested he may have posed as a girl on Facebook and posted information about a fake McDonald's promotion to lure people to the fast-food restaurant where he first opened fire.
Munich police chief Hubertus Andrae told a press conference Saturday that there were “no indications whatsoever” that Sonboly had any links to the Islamic State group or that the killings were motivated by politics.
Andrae did acknowledge that Sonboly had extensively researched the theme of rampages and may have read about the lethal killing spree by white supremacist Anders Behring Breivik, saying there was an “obvious link” between Breivik's crimes and Friday's shooting.
The carnage came on the fifth anniversary of right-wing fanatic Breivik's massacre in Norway that killed 77 people.
Sonboly had reportedly received psychiatric treatment in the past as he battled with depression.
Neighbours say he was born to Iranian parents — a taxi driver father and a mother who worked at a branch of the Karstadt department store. He also had a brother.
They have yet to be interviewed by police “because of their state — they also lost their son,” said the Munich prosecutor.
The family lived together in a tidy, recently-built social housing block that is mostly home to immigrant families, located next to a luxury Maserati car showroom in the well-heeled Maxvorstadt neighbourhood.
Just 10 minutes' drive away from the mall, the area has been deluged with journalists in the wake of the shooting.
Sedik Ali, a 29-year-old neighbour originally from Afghanistan, remembered Sonboly as a lonely teenager who rarely hung out with the other neighbourhood lads.
“It's strange, but he never spoke with us,” said Ali, who regularly played football with the gunman's brother in a nearby park.
Delfye Dalbi, 40, a neighbour originally from Macedonia, described him as “a good person”, like many in the area remembering that he had a job distributing free newspapers.
“I know this boy very well — he always was here to help,” she said.
“He would give you the newspaper, he was nice. He was never bitter or angry. I feel so sorry for this family but also for the boy.”
She added: “I never heard of him having problems with the police or other neighbours.
“He laughed like a normal person.”
But a video posted on social media appeared to cast Sonboly in a different light, showing a man dressed in black walking away from the McDonald's restaurant while firing repeatedly on people as they fled.
Survivors described terrifying scenes as shoppers rushed from the area, some carrying children in their arms.
Another video posted online appeared to show Sonboly on the top floor of a multi-storey car park exchanging a tirade of insults with nearby bystanders.
At one point the gunman appears to mention being a victim of bullying though his words cannot be heard clearly.
“I'm German, I was born here,” the assailant is heard to reply after one man reeled off a volley of swear words, including an offensive term for foreigners.
At another point, he says: “I was in hospital.”
During the heated exchanges, a bystander on the balcony cries out: “He's got a gun here, the guy has one.”
A police source cited by DPA news agency said Sonboly loved playing violent video games.
He was also an admirer of Tim Kretschmer, the 17-year-old German who shot dead 15 people at his school near Stuttgart in 2009 before turning the gun on himself, the source added.
“All his body language said 'I don't want to talk to you,'” remembered Stephan, a waiter at the trendy Treemans cafe on the ground floor of Sonboly's housing block.
“He wasn't like other guys his age — cool, full of energy, with the latest haircuts. He seemed calmer. He was a shy boy.”