I have to agree - this attack was long in the planning.
As for stepping up and taking responsibility...
Its just too hard to know -
stick and move is the way to beat a bigger, stronger opponent - stay mobile and don't present a visible target for retribution, but,
it is VITAL for terrorists to be seen as martyrs and "sticking it to the man" so to speak, even when it raises the risk of being hammered - becuase lets face it, the trouble is that even if we know the group who did it, they can just disperse and hide in friendly countries and we still can't find them.
Provided that this attack was not planned, executed or supported by a major visible country/regime - provided that the group responsible is small, fragmented and mobile, I really don't think they have much to lose or risk any greater danger by taken credit.