@hedge
No group or individual has so far claimed responsibility.
This has been mentioned a few times, but I think you're remembering 1970s/80s IRA SOPs and imagining that this is how Islamic extremists act.
When did anyone claim responsibility for 9/11? Or the Bali bombings? Or 7/7? Bin Laden mentioned 9/11 in one of his taped messages, much later, but never said he was responsible. Al-Qaeda didn't claim responsibility for the July 7 bombings until September, and intelligence sources reckon that was false anyway. The men convicted of the Bali bombings went to their deathbeds denying responsibility.