At the moment no organisation or person has claimed responsibility, though as the reports mentioned above state, there are some obvious suspects.
This morning I listened to the story unfold on ABC radio which relied upon several Indian reporters for most of its stories (doesn't the ABC have a correspondent on the spot any more?). Generally the reporting was good, with one exception: a person in New Delhi who seemed to be getting her information third hand. This shows the value of having people with local knowledge on the spot, something which the Australian media should bear in mind about local news coverage. When a disaster occurs, whether natural eg cyclone or unnatural eg bombing radio is probably the best way to keep people in the vicinity in touch with what's happening or likely to happen.