Four bombs exploded at a Madrid train station at rush hour this morning killing 170 people. Who's responsible?

There was no immediate claim of responsibility for the blasts, but Spanish officials said they suspected militant Basque separatists from the group E.T.A., whose initials stand for Basque Homeland and Freedom in Euskara, the Basque language. E.T.A. was placed on the United States's list of terrorist organizations in 2001 because it was determined by the State Department to have committed, or to pose a significant risk of committing, acts of terrorism. "I do not have the least doubt that it is E.T.A.," said the interior minister, Ángel Acebes, in a statement broadcast on the radio.

Not so fast?:

...but the latest attack was on a scale far larger than anything attributed to ETA in the past. For that reason, some commentators have speculated that other terror groups, including al Qaeda, may have been involved. Some fear that Spain's strong support for the United States in the run-up to war with Iraq could make her a target. This was picked up by the leader of the banned Basque separatist party Batasuna, Arnald Otegi. He rejected claims that ETA could have been behind the attacks and instead pointed the finger at "the Arab resistance".

Here's more on reaction in Madrid to the bombing. The death toll is now at 186, and undoubtedly going to climb. UPI reports on the bombing as the possible hallmark of Al Qaeda.

We're back at work today, so here's an open thread to discuss it.

Update: The mystery deepens:

Police probing the Madrid terrorist attacks found a van with detonators and an Arabic-language tape with Koranic verses, and officials said they are not ruling out any line of investigation.

[comments now closed, new thread here]