It can't be the ETA...
The evidence points more and more against AL-Qaida.
As Master Gunner said, the ETA does almost always take the credit, and most of their bombs are harmless, just to show the Spanish government that they can do it..
I heard on the news now too that the explosives that was used was not of the same type as the ETA almost always use. (they stole about 8 tonnes of the same type some years ago..)
But the Spanish government still claims that ETA could have done it.
Seems like they are using this tragedy and try to hurt ETA's cause now.