Sonboly's rampage at a Munich shopping mall on Friday sparked a terror alert, with fears that Germany had followed France and Belgium this year in becoming targets of the Islamic State group.
But after a forensic sweep of Sonboly's home, investigators on Saturday ruled out any link with the ruthless jihadists.
Using a 9mm handgun, the 18-year-old German-Iranian shot dead nine people, most of them fellow teenagers, before killing himself with a shot to the head.
Thirty-five others were injured, 11 of them seriously, according to a new toll released by Munich police on Sunday. Those figures included people who hurt themselves while fleeing.
While investigators have ruled out a link to IS, their probe has turned up another dark scenario—of a violence-fixated youth who tempted his young prey to their fate via the internet.
Sonboly probably hacked a girl's Facebook account and used it to lure victims to a McDonald's outlet where they expected to get vouchers for price reductions, Interior Minister Thomas de Maiziere said, describing it as "particularly underhand."
But if a clearer picture is emerging of how Sonboly planned the killing, his motive remains unclear.
Of Shiite Muslim origin, Sonboly appears to have converted to Christianity, hence his first name David.
Investigators describe Sonboly, who lived with his parents in social housing, as a depressive obsessed with shooting sprees and a devotee of violent video games.
They found documents about far-right fanatic Anders Behring Breivik who murdered 77 people in Norway in 2011—a massacre that occurred exactly five years to the day before the Munich shootings.
But what drove him to commit a mass killing?
Neighbors said Sonboly was born to Iranian parents, a taxi driver father and a mother who worked at a department store. They arrived in Germany as asylum seekers in the late 1990s.
The family lived in the well-heeled Maxvorstadt neighborhood in a tidy social housing block that is mostly home to immigrant families.
One idea put forward by the mass circulation newspaper Bild suggests Sonboly had been bullied by Turks at school, and wanted to take revenge against foreigners.
The dead included three Turks, two of whom had dual German nationality, a Hungarian, a Kosovan, a Greek and an individual who was stateless, according to the latest figures.
Video footage from Friday also apparently shows Sonboly on a car park roof in a heated exchange with a man on a nearby balcony.
"I'm German, I was born here," the assailant replies after the man swore at him, using curse words for foreigners.
Another question is how Sonboly acquired the Glock 17 handgun—By coincidence or not, a type also used by Breivik—whose serial number had been filed off.
A debate is already underway as to whether Germany's gun laws, which are already strict, should be tightened further.
Vice Chancellor Sigmar Gabriel, in an interview with the Funke press group, called for a maximum effort to "restrict access to lethal weapons and monitor it closely."
European leaders swiftly voiced solidarity with Germany as the terror alert was launched—a sign of the jittery mood after a string of jihadist assaults.
The attack came just four days after a 17-year-old asylum seeker went on a rampage with an axe and a knife on a train in Bavaria, injuring five people. He was believed to be a "lone wolf" Afghan or Pakistani inspired by Isis.
And in occurred just over a week after a Tunisian used a truck to mow down 84 people after a Bastille Day fireworks display in Nice, the third major attack on French soil in the past 18 months.
Isis described Nice gunman Mohamed Lahouaiej Bouhlel as one of its "soldiers," though investigators have not found direct proof of his allegiance to the jihadists.
The Eiffel Tower in Paris was lit up in the colors of the German flag late Saturday in tribute to the Munich victims.