The gunman who killed nine people in Friday's Munich shooting rampage appears to have bought his gun illegally on the internet and planned the attack for at least a year, investigators said on Sunday.

The shooter, identified by an official as 18-year-old Ali David Sonboly , used a reactivated pistol he probably bought illegally on the so-called dark net, investigators said, offering an initial explanation of how a teenager who had been treated for mental-health issues was able to access a firearm undetected.

The shooting spree, in which Sonboly killed nine people before shooting himself, sent shock waves through a country on edge. Germany hasn't faced a major terrorist attack, but the influx of a record number of migrants last year and repeated terror attacks in neighboring countries have left residents worried.

On Sunday evening, Munich police took into custody a 16-year-old Afghan boy who was a friend of the shooter. Police said investigators had interviewed the boy on Friday and later discovered contradictions in his statements, raising suspicions that he had known about the attack in advance.

Friday's attack was meticulously planned by a young man with mental-health problems who drew inspiration from previous mass shootings, officials said.

"From documents we have examined thus far, it seems he had been planning his act already since last summer," said Robert Heimberger, chief investigator at Bavaria's criminal investigative agency, at a press conference.

The gunman last year visited the site of a 2009 school shooting in the German town of Winnenden, where 17-year-old Tim Kretschmer killed 15 people with a gun he stole from his father.

Investigators, who found photos of that visit on Sonboly's digital camera, believe he began planning his attack after the trip. "He visited and photographed the scenes of the crime there and delved very concretely into these mass killings," Mr. Heimberger said.

Munich Police Chief Hubertus Andrä over the weekend also drew parallels between Sonboly's shooting spree and the 2011 attack in Norway, when Anders Behring Breivik killed 77 people, mostly teenagers. Mr. Andrä noted the apparent targeting of young people in both the Munich shooting and Norway's deadliest mass murder. Seven of the nine victims in Friday's shootings were teenagers. Authorities said the shooter was likely inspired by Breivik.

Documents investigators recovered from Sonboly's bedroom included research related to the Norwegian attack and a German-language version of the book "Why Kids Kill: Inside the Minds of School Shooters," investigators said on Saturday.

By examining Sonboly's computer, Mr. Heimberger said investigators have traced a chat-room conversation that led them to believe he probably bought the pistol, which had been deactivated, on the dark net—a sort of parallel internet that traffics in illegal goods such as weapons and drugs, often used by criminals.

"I'm personally not getting on the dark net but I am noticing that many teenagers are able to get on it," Mr. Heimberger said. "I'm convinced that there are many possibilities in chat rooms to find out how to get in there."

Mr. Heimberger said police believe the weapon might have come from Slovakia because it had a Slovak certification mark. Police said separately that the believed the weapon was illegal because its serial number had been scratched off.

Mr. Heimberger cautioned the investigation into the gun was just beginning. Slovak officials on Sunday said they hadn't been contacted by German authorities about the gun but were prepared to help.

Sonboly also carried more than 300 rounds of ammunition.

Reactivated weapons have come into focus in Europe, after they were found to have been used in at least two terror attacks, including the assaults on the Paris office of satirical magazine Charlie Hebdo in January 2015.

Deactivated weapons have long been seen as a potential loophole in Europe's gun-control laws, and law-enforcement officials have pointed to instances where decommissioned weapons from Slovakia have been sold on their streets.

In Germany, which has among the strictest gun laws in the world, officials were quick to call for them to be tightened even more in the wake of the Munich attack. German Interior Minister Thomas de Maiziè re told German daily Bild am Sonntag on Sunday that the government would determine if a change to gun rules was necessary once it became clear how Sonboly got the gun.

Gun laws in Germany were strengthened in 2003, a year after a 19-year-old student killed 16 people and then himself at a high school in Erfurt. The law requires gun owners to register and securely lock their weapons.

Criminal experts said Germany doesn't have much room to tighten gun rules and that even if it did, it likely wouldn't impede disturbed, lone gunmen.

"In Winnenden, the shooter had the [legal] gun from his father.…In Erfurt, the shooter had a gun license," said Marc Coester, professor of criminology at the Berlin School of Economics and Law. "When you have someone like that, he can use a truck like in Nice," he said, referring to the terror attack in Nice that killed more than 80 people this month.

Investigators said Sonboly's motive remained unclear, but excluded revenge against specific classmates for past bullying or targeting foreigners.

Sonboly suffered from depression and panic attacks when among others, and in 2015 he was treated for two months in a psychiatric hospital, Mr. Heimberger said. Investigators also found medication, which they didn't identify, in searches of Sonboly's room, but it was unclear if he had been taking it.

In May, Sonboly created a fake Facebook profile, using a real girl's details, that he used on Friday afternoon to lure people to the McDonald's, investigators said.

Sarah Sloat, Georgi Kantchev and Anton Troianovski

Write to Ruth Bender at Ruth.Bender@wsj.com and Christopher Alessi at christopher.alessi@wsj.com

 

(END) Dow Jones Newswires

July 24, 2016 21:25 ET (01:25 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.
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