[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.

[Fedora]

I think you're probably right that if ETA was involved it was through that type of resource-sharing rather than the actual planning or execution--this would explain why some pieces of data point towards ETA involvement while others point away from it, which would be the case if Al Qaeda got ahold of ETA resources and then used them to carry out an operation not in ETA's interests.