Rumours of the responsibility of the Hezbollah, or Israel depending on which side of the fence you are, for the blast has been going on since day one, and there hasn't been any substantial proof.
Don't get me wrong, I have no sympathy for the Hezbollah, but lets be honest, the tragedy can be very well explain by the sheer level of incompetence and corruption of the Lebanese government (in which the Hezbollah also plays a big role).