A very sad day for the people of Spain. I hope they can find the people responsible for this and make them pay dearly for what they have done.

www.cnn.com/2004/WORLD/europe/03/11/spain.blasts/index.html (http://www.cnn.com/2004/WORLD/europe/03/11/spain.blasts/index.html)

It's pretty much guaranteed to be ETA, they've commited terrorist attacks for years now, though none on this scale. I think they killed about 20 in Barcelona a while back?
Anywho, back at christmas you may remember that the Spanish police intercepted several bombs on trains headed to Madrid, which has been the pattern of ETA in previous attacks (using trains).
The only thing that deviates from the suggestion that it is them, is that they are never really out to "kill as many people as possible", they are much like the IRA in that it's purely for show, as they usually make the proper authorities aware of any attack before it happens, which makes this instance a little odd.
Many people here though think it's more of a rogue section of ETA, probably idealist youths of the faction.

Unfortunately, the ETA problem has existed for a long time, and these attacks are nothing new, though the scale is larger than usual. Spain - and not the EU - needs to come to terms with the faction and either A) tackle it or B) come to some arrangement with the voice of the faction. They want independence, well then something needs to be done. Either way, more blood will no doubt be spilt from those who have nothing to do with it.

It's not just Spain that has to deal with them, it's France too. I would imagine both countries need to work closely to put the fanatics responsible behind bars. That is where the EU might help...in coordinating efforts and making sure there are no safe havens just across a border.