The 18-year-old gunman who killed nine people in a mass shooting in Munich on Friday spent more than a year planning the attack and was able to buy a handgun on the dark web, investigators have said.

Bavarian investigator Robert Heimberger said Ali Sonboly had visited the scene of a previous school shooting in the German town of Winneden and took photographs, adding further evidence to the claim by Munich’s police chief, Hubertus Andrae, that the teenager was “obsessed with shooting rampages”.

Investigators also found on his computer photos of Anders Breivik, who killed 77 people in Norway in 2011. The Munich gunman planned his attack on the fifth anniversary of Breivik’s shooting and used a similar Glock 17 pistol.

Authorities also confirmed that the gunman had written a manifesto before the attack, but did not reveal any details about its content.

The shooter’s identity was confirmed to police on Friday by the father of the gunman, who recognised his son on a video that was circulating on social media after the shooting and went to the police, Heimberger confirmed.

Heimberger said it is likely the gunman purchased his illegal weapon online, through a website trafficking illegal weapons hosted on the dark web, an area of the internet that allows users to remain anonymous and is often used for illegal purposes. Police came to its conclusion after observing messaging history on the gunman’s computer.

Authorities have confirmed the weapon is an originally deactivated version for use in theatres, which was subsequently reconverted to fire live ammunition. Similarly reactivated weapons had been used in the Charlie Hebdo shooting in January 2015 in Paris.

They also confirmed the weapon used in Munich bears a proof mark from Slovakia in 2014. A proof mark is added to the barrel of a gun after a stress test is conducted to ensure it can be fired without damage to the barrel.

The Bavarian interior minister, Joachim Herrmann, confirmed that the gun cost “several hundred euros” and authorities are trying to work out if Sonboly was able to buy the gun with money earned from his paper round, which was his sole income.

Hans Scholzen, a German weapons expert, told the Guardian that similar deactivated weapons can be legally bought for €200 (£167). “But a weapon officially deactivated in Germany cannot be reconverted to fire live ammunition without destroying the gun barrel,” he said. “Thus it must have been deactivated sloppily and in a country that does not monitor such adjustments properly.”

These revelations came as the German interior minister, Thomas de Maizière raised the possibility of tightening gun control laws in the country.

Thomas Steinkraus-Koch, spokesman for Munich prosecutor’s office, said there was still no evidence of any political motivation to the crime, nor that the shooter targeted specific victims. A police spokesman also confirmed that the suspect spent two months having inpatient psychiatric treatment last year. After leaving hospital, he continued to receive outpatient treatment for social anxiety disorder and depression for which he was receiving medication.

Investigators also confirmed the teenager appears to have attempted to lure his victims to a McDonald’s restaurant, the initial site of the shooting, with a message on a fake Facebook page which promised free meals to anyone at the venue at 4pm.

Police believe this was a venue Sonboly knew and they initially thought he may have recognised his victims, although he did not begin shooting until two hours after the Facebook invitation.

Sonboly had spent time researching previous mass shootings, including one in Winneden, a site he visited and photographed. In 2009, Tim Kretschmer, 17, killed 15 people at his former school before fleeing and killing himself.