This might have been a provokatsiya by the Serbs (or the Russians)
http://20committee.com/2014/09/14/is-islamist-radicalism-rising-in-serbia/
http://20committee.com/2014/03/29/understanding-provocation/
http://20committee.com/2014/09/25/what-if-everything-you-know-about-terrorism-is-wrong/
Thanks AdmSmith.