The full name of the killer is Sanel Mahmutovic.
He is your Bosniak Muslim brother from the town called Sjenica. Sjenica is in the Sandzak region of the south-western Serbia. There are few ethnic groups in Serbia that are predominantly Muslim, one of them are Bosniaks from Sandzak. They self-declare as Bosniaks since 90s, before that they were simply called Muslims (as ethnicity). In the region they are easily recognized by their typical names and surnames.
@Engineer
This is the source that the killer is from Sjenica. It's in German, you can use translate service to check. Change xx to tt.
hxxp://primavera24.de/nachrichten/gelnhaeuserin-22-kaempft-weiter-um-ihr-leben/96120/attachment/191114-gelnhausen-tugce-haehme-foto_screenshot-facebook
hxxp://20min.ch/panorama/news/story/21220216