They're calling it Spain's 9/11:

There was no advance warning of the attacks. At first, the Spanish authorities blamed E.T.A., the Basque group that has been seeking independence from Spain for more than three decades.
Later today, however, the Interior Ministry said the police had found a van with detonators and an Arabic-language tape of Koranic verses, according to news agencies, and that it was considering all lines of investigation.
An Arabic newspaper, Al-Quds al-Arabi, said it had received a claim of responsibility for the train bombings issued in the name of Al Qaeda.
The five-page e-mail claim, signed by the shadowy Brigade of Abu Hafs al-Masri, was received at the paper's London offices. It said the brigade's `death squad" had penetrated "one of the pillars of the crusade alliance, Spain."

In my mind, it's senseless and despicable no matter who committed Those who did it are solely responsible and their is no guilt to be determined outside of who those people are. In trying to imagine it from the perspective of a Spaniard, I can see how the question of "who to blame" comes to mind, and I can imagine how, if it turns out to be al Qaeda, some people will say Spain should have left well enough alone and not stood with us. I can only say that I understand and appreciate that Spain has had a choice (and taken risks) in standing beside the US more than most countries have after 9/11. I believe that choice was the right one, no matter how difficult it was, and I hate to see that they are suffering for making the right choice. But that being said, I hate to see this all around. Regardless of who did it, it was undeserved and evil.

Today something horrific happened in Spain. When will people wake up? When will people understand that there can be no justification for terrorism?

Please listen. This is an epidemic. It may seem far away from you now, but it will reach you eventually, wherever you are. If you do nothing to stop it now, you are bringing it nearer. Before you know it, the little girl on the television will be speaking your language, will be someone you know.

At a Spanish demonstration protesting today’s terrorist attacks we find a perfect example of the sick hypocrisy that is poisoning Europe, as a useful idiot in knit cap and kaffiyeh proudly aligns himself with the symbol of Palestinian terrorism.

Como acto de repulsa y condena del brutal atentado en Madrid y en
solidaridad con las víctimas y sus familiares, el día 12 de marzo a las
13 horas, coincidiendo con las manifestaciones convocadas en toda España,
habrá una concentración silenciosa frente a la sede del Instituto
Cervantes en Nueva York (211-215 East 49th Street).

Juan Manuel Egea
Cónsul General

Consulado General de España en Nueva York

Aunque el Gobierno sigue pensando que ETA está detrás de la masacre perpetrada en Madrid, no descarta que un grupo terrorista árabe pueda ser el responsable de los atentados que han costado la vida al menos a 192 personas y ha causado heridas a más de 1.427. La policía ha localizado en la localidad madrileña de Alcalá de Henares una furgoneta robada en la que se han hallado siete detonadores y una cinta en árabe con versículos del Corán.