Some group claiming to be al-Qaeda has claimed responsibility for the attacks. Which could well be true; initially it didn't look quite spectacular enough to be them, though it could well be a group of radical Islamist copycats; a franchise operation, as it were. Or a feint to draw out the emergency response services and analyse their plans in preparation for a later, more devastating, attack. It's probably not the IRA, anti-globalisation groups, animal-rights militants or anyone else.
Then again, after such an attack, nutters usually come out of the woodwork to claim responsibility and get some free publicity. The militant Islamist group who claimed responsibility may or may not be connected to whoever actually carried out the attacks.