To add a bit with no response required. Whoever carried out the attack was very carefull not to damage the tombs of the two caliphs. Only the dome was taken out. If it was done by the sunni to spite the shia they would have done a far more complete demolition. This job was quite surgical in nature.
Yes I know. And no neighboring structures were damages. Al Queda is never so surgical or precise. They do as much damage as they can. So all signs point to Iran.