They may well be as claimed by Turkish security:
Police said they believed the attackers were members of a Turkish Sunni fundamentalist group, the Great Eastern Islamic Raiders Front.
... or they may not. There've been a number of instances of groups being slow to claim responsibility and that smacks of a broader destabilization plan. I always think one must look at the net effect of such a thing - whose attitudes does it harden and why would anyone want those attitudes hardened?
In other words, who are all the groups or countries who stand to gain overall?
I can't see the killing itself as the main object. On the other hand, there is a type of law-unto-himself person who just gets a kick out of seeing the big explosion. The sniper killings in the U.S. spring to mind here.
Might well be wrong.