Analysis: Who bombed Madrid?
(CNN) -- Authorities in Spain are continuing to investigate who carried out the near-simultaneous bombings in Madrid that killed about 200 people. CNN's Senior International Correspondent Sheila MacVicar looks at who may be to blame.
Spanish Prime Minister Jose Maria Aznar and other investigators who were so adamant on Thursday that it had to be ETA are now saying every line of inquiry will be pursued.
That change of course comes about, at least in part, by the discovery on Thursday of a van near one of the train stations, containing some Arabic verses and detonators that may be the same as those used in Madrid.
Spanish police are also examining a bomb-carrying backpack found in the wreckage of one of the trains. This may offer the most important clues as to who is responsible.
The Spanish newspaper El Mundo is reporting that inside the back pack, investigators have found a packet of explosives, a cable connected to the explosives, and apparently loose in the bag, a mobile telephone.
Explosives have chemical "signatures," and police will want to know whether the explosives used Thursday come from a batch of dynamite stolen in France in 1999.
Some of that dynamite has been used by ETA in the past, and it is believed the remainder of the dynamite is under the control of ETA.
If analysis finds a match, that would offer very powerful evidence that ETA, or an ETA faction, was responsible for Thursday's attacks.
Without that conclusive link, and without a credible claim of responsibility, investigators will want to continue to pursue all other tracks.
Meanwhile, a group claiming to represent al Qaeda has said it was behind the attack, although intelligence sources have consistently told CNN that the group does not speak for al Qaeda, and is unreliable.
The group calling itself Abu Hafs al-Masri Brigade has made false claims before for other actions though, including for example, the U.S. power blackouts last summer that turned out to be caused by technical problems and not an act of terror.
What do we know is that if this was indeed ETA it would represent a dramatic shift from what we have seen from them in the past: In terms of tactics and targeting; in terms of the way it was carried out; the indiscriminate targeting of civilians with no warning and no claim of responsibility.
Terror organizations, as bizarre as this sounds, act within their own boundaries, to their own rules, and have their own justification for how they operate and who or what is an acceptable target.
Thursday's attacks broke every one of the rules that ETA has operated by in the past. So if this was ETA, the attack marks a fundamental shift, a passing of one guard to another, according to some Spanish officials.
In effect it would be such a paradigm shift it would almost be a new organization.
U.S. officials say they suspect a group linked to al Qaeda and a link between ETA and some Islamist extremist group cannot be ruled out.
Ultimately though, they have very different goals: ETA's is ostensibly a political struggle for an independent Basque homeland being waged through terrorist means while al Qaeda wants to cause chaos to the West with no possibility of negotiation.
What is more likely is that rather than direct assistance between the two groups, one can see how there can be a cross-pollination of ideas.
You perhaps have young radicalized members of perhaps an ETA faction looking at the experience of other terrorist acts elsewhere. They could be learning from that experience; looking at the Bali bomb for example, and at how other groups have sought to amplify these acts of terror, by indiscriminate killings and by failing to give warnings.
These are things we haven't seen from ETA in the past; so perhaps younger new members of a radical ETA faction are picking up some of the lessons of other attacks they have seen elsewhere.