[Fedora]

Good points. Also, another terrorist group--whether Al Qaeda or otherwise--might cultivate a contact in ETA in order to use that group's resources, without ETA as a whole being involved. Lots of possible scenarios to consider there.

[Prodigal Son]

Oh yeah. That's exactly how I imagine it works. This one has a contact here and there. They're all 'networked' in this manner which is exactly why they are called a network in the first place.

It's just like the black market and selling drugs. This guy will know someone who can get this or that done, where this can be bought and so forth or put you in contact with someone who can.

[ScaniaBoy]

Yes, a possible scenario is that a muslim terrorist group has made a deal with ETA (or some splinter group) to use their local resources. ETA has declared a "truce" in Basque and Catalonia but not in Andalusia (where Madrid is situtated).

However, they might not have had anything to do with the actual planning and carrying out of the deed, and now they are frantically trying to back-pedal, because if this outrage will be tied to ETA they will loose the last smidgen of support they have.