On the first read, it looks like a very convincing reasoning. But it falls short of facts in many respects.
Its highly unrealistic to think that this story was found our just few days after blast. They were able to track people who left India and settled down in Pakistan, moved to UAE and come back in India illegally. Its not a one-day affair and these events would have been tracked for months, if not years. Why couldn't pre-emptive measures taken, if they already knew about it.
Either these agencies are too inefficient to handle terror attacks (or) they are just being creating to bring some story out.
Only God knows the truth. If what they claim is true, they should provide convincing proof to international community and extradite the culprits and punish them in the harshest possible means.