The teenage killer who gunned down nine people in the German city of Munich had an "obvious link" to the Norwegian mass murderer Anders Behring Breivik, police said. They also warned that they expect the numbers of dead and wounded in the attack to rise.

Police did not elaborate on the connections between the two mass killers, so it is unclear whether the Munich attacker, named in local media reports as Ali David Sonboly, held similar right-wing beliefs as Breivik or was simply inspired by his rampage in 2011.

Munich police chief Hubertus Andrae told a news conference the suspect's motive was still "fully unclear", but that German prosecutors said that they had found written material about "shooting rampages" in Sonboly's room at his parents' home – including a book called Rampage in Head: Why Students Kill. Investigating officers have described the 18-year-old as being "obsessed" with mass shootings.

It is not known whether Sonboly, described by police as being of German-Iranian origin, held similar right-wing beliefs as Breivik. Although the Munich attack took place on the fifth anniversary of Breivik's attacks, and Sobboly's classmates told German tabloid Bild that he used an image of Breivik as his profile picture on the messaging service WhatsApp.

Officers are also investigating whether he lured his victims to their deaths using a fake Facebook account under a girl's name "Selina Akim". It is thought he may have used it to invite people to the McDonald's restaurant, tempting them with offers of free food before attacking.

Mental illness assumption

A spokesman for the Munich prosecutor, Thomas Steinkraus-Koch, said Sonboly might have been under psychiatric care, and had been the victim of violence himself.

"We are assuming that he may have suffered from depression," he said. "As far as we know he has no criminal record. In 2012 and 2010 he was a victim of an attack – on one occasion he was beaten by three young offenders."

Munich police have warned the number of injured could increase, when people who fled the scene of the attack come forward. Ten people are currently critically ill, including a 13-year-old boy, he said. Among the dead were three Turkish citizens and three from Kosovo, their respective government officials said.