The teenager behind Friday's deadly shooting rampage at a Munich mall had planned his attack for a year and chose his victims at random, investigators said.
Bavarian investigator Robert Heimberger said the shooter, 18-year-old David Sonboly, visited the site of a previous school shooting in the German town of Winnenden and took photographs last year, then set about planning the attack in which he killed nine and wounded some three dozen others before taking his own life.
"He had been planning this crime since last summer," Heimberger told reporters.
He said there were "many more terabytes" of information to evaluate, and that the teenager's brother and parents were still not emotionally up to being interrogated by police.
There is so far no evidence that he knew any of his victims, or that there was any political motivation behind the attack, said Thomas Steinkraus-Koch, a spokesman for the Munich prosecutors' office.
The suspect received both in-patient and out-patient psychiatric treatment last year to help him deal with "fears of contact with others", Steinkraus-Koch added.
He said medication had been found at his home but that investigators needed to talk with his family to determine whether he had been taking it.
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Weapons are strictly controlled in Germany and police are still trying to determine exactly how the shooter obtained the Glock 17 used in the attack.
Heimberger said it appears "very likely" that the suspect purchased the weapon online through the so-called "darknet". It was a pistol that had been rendered unusable and sold as a prop, then was restored to a fully functioning state.
Its serial numbers were filed off and Sonboly had no permit to purchase weapons, authorities have said.
Even though it was an illegal weapon, Vice Chancellor Sigmar Gabriel suggested on Sunday that even stricter controls on legal access are needed, telling the Funke Media Group that "we need to do everything further possible to curb the access to deadly weapons and strictly control them".
"How can it be that an unstable, or possibly even mentally ill 18-year-old comes into possession of a firearm?" he asked.
Germany's top security official, Interior Minister Thomas de Maiziere, said, however, that Germany's weapons regulations are already "very strict" and appropriate.
He said authorities first need to determine where the shooter got the weapon from.
"We'll then have to look very carefully at whether there then needs to be a possibility for additional legal action," he said.