Good question. The attack has no Islamic characteristics, as you can read here:
3/11: The Non-Existent Islamist Character of the Atrocity
ETA did participate in 3/11, although not as perpetrator, but in a previous disinformation campaign, as you can read here:
3/11: The Disinformation Campaign
I think it was an inside job, probably carried out by mercenaries under cover of officials of the Spanish security forces. Surprisingly, Frank Gaffney Jr. pointed out something very similar before we knew a lot of evidence suggesting this, as you can read here:
Spains Terrorgate?