David Ali Sonboly — the 18-year-old gunman who killed nine people in Munich — was depressed and had spent two months in a psychiatric unit last year, police have revealed.
The teen who had German and Iranian nationality, was obsessed with mass killings and spent a year preparing for the shooting spree, police said.
At least 35 people were also wounded during Sonboly's attack, which began at a McDonald's outlet and ended with him turning his 9mm Glock pistol on himself.
Investigators have ruled out any link with Islamic State jihadists, although he appeared to have planned the assault with chilling precision for a year.
Police have also arrested a 16-year-old Afghan friend in connection with the shooting.
The youth was under investigation for possibly having failed to report the plans of the gunman, a police statement said.
"There is a suspicion that the 16-year-old is a possible tacit accomplice to (Friday's) attack," it said.
The Afghan youth was questioned after he contacted police following the shooting on Friday, the statement said.
Investigators later uncovered contradictions in his statements, it said, without providing any further details.
Germany in shock
Hundreds of people, many of them in tears, gathered on Sunday outside the Munich shopping centre where the attack took place to pay tribute to the victims.
Already steeped in grief and shock, Germans were further rattled by news that a Syrian refugee had killed a 45-year-old Polish woman with a machete in the south-western city of Reutlingen.
Police said that incident on Sunday, in which three others were injured, did not bear the hallmarks of a "terrorist attack".
"When a man and woman have an argument, we assume that we are dealing with a crime of passion," a local police spokeswoman told DPA.
Three people were also injured in the attack, which ended when the 21-year-old assailant was hit by a car.
NTV showed amateur video footage of the suspect running away from the scene before cutting to him lying on the ground, his face bloodied and his hands cuffed by police.
A 27-year-old Syrian man denied asylum in Germany a year ago died on Sunday when he set off a bomb outside a crowded music festival in Bavaria, the fourth violent attack in Germany in less than a week.