The detail was captured in a video of the gunman driving to the scene of the Christchurch shootings.
The song playing in the gunman’s car was from a propaganda video produced by three Bosnian Serb soldiers and warns Bosnian Muslims that Serbs, led by Radovan Karadzic, were coming for them.
The song has spurred a multitude of parodies and has become an internet meme called ‘Remove Kebab’, a euphemism for ethnically cleansing Muslims, Al Jazeera reports.
The phrase ‘Kebab removed’ was written on the gunman’s rifle.
The suspect was also believed to have been inspired by other historical figures who fought Muslims, Al Jazeera says.
The gunman claimed in a 74-page manifesto published before the attacks that he had been inspired by Anders Breivik, the Norwegian white supremacist who murdered 77 in an attack in Norway in 2011.
Political scientist Jasmin Mujanovic wrote on Twitter that the markings were "steeped in toxic, faux-historical narratives about 'defending' white Christendom".