Pakistan has said that an investigation by the ISI and the IB has already started to see if there was a group behind the killing or just an individual act. Letting the ISI investigate this is like letting the wolf guard the hen house. Of course nothing will be found unless of course they decide to pin it on the TTP which would make since considering that groups fondness for making war against the Pakistani government. I do not see this going much past this was an individual act.
Update as I continue to read about this case. There may have been at least another person involved. As I said above I will be stunned if one of the good Taliban groups is accused of being involved it will probably be linked to the TTP.