The Spanish government (there are elections in Spain, tomorrow) wants the spaniards and the world to believe it was the ETA (that way they will vote for law and order and essentially for the Popular Party at the government).
But if it's Al Qaeda, this is going to be very damaging to the government: first it will give credit to the ETA (which denied it did it), then most importantly, it will be seen as the consequence of Spain's involvement in Iraq.
Do not forget the overwhelming majority of Spaniards were AGAINST the war in Iraq, and now the Spanish people are themselves victims of something they didn't want in the first place.
Asnar would face a terrible bloody legacy.
But it's true Spain is performing very well in the EU , so other considerations will appear on the elections.

Wasn't Bush's purpose in invading Iraq to eradicate terrorism?
Didn't the antiwar say it will only exacerbate it?
Look who's right!

Raoul