It is still unclear who was responsible for the attacks in Madrid after the Spanish authorities announced they had arrested five suspects in connection with the blasts.
At first, Eta was blamed for the carnage
Some in Spain argue the attacks may be connected to the work of Eta, while others say the Basque separatists could not have planned attacks on such a huge scale.
BBC Security Correspondent Frank Gardner looks at what we know so far about who could be responsible.
Could this have been a breakaway group from Eta or even the result of a collaboration between Eta and al-Qaeda?
Counter-terrorism experts around the world are looking at the possibility that there was a kind of collusion between a violent, fanatical, splinter group of Eta and extremists from outside. There have previously been connections between Eta and the GIA, an Algerian militant group.
Normally al-Qaeda would not work with non-Muslims, as they do not trust them.
But al-Qaeda is not a fixed organisation, it is a shadowy phenomenon, and it is possible that there are people sympathetic to them, who have worked with them.
Al-Qaeda also has form in Spain. They held their final eleventh-hour terror summit in northern Spain in July 2001, where Mohammed Atta, the main September 11th hijacker, briefed the others and sent news to Afghanistan about the imminent attacks.
What are the experts and governments saying about who was responsible?
The UK government is saying officially that it takes its lead from the Spanish investigation, but I can say that privately officials are holding their breath, hoping this is not al-Qaeda.
If this is a Spanish issue, dreadful as that may be, at least it would be contained there.
In the UK, the Joint Terrorism Analysis Centre inside MI5 is taking in all the input so far. It will be compiled into a report for the Joint Intelligence Committee and will be passed to the prime minister and parts of government.
What points to al-Qaeda?
The Spanish government is keeping an open mind and since the investigation is only just underway, that is probably the best thing to do.
There is an analysis going on into the authenticity of the claim sent to a London newspaper office hours after the bombing, claiming that the attacks were the work of al-Qaeda.
There are several factors that point to them. Firstly there is the discovery of a van containing bomb detonators and Koranic chants. The second is the claim of responsibility from an obscure group in Canada, but we can perhaps dismiss that, but the most important is the claim sent to London saying that they did it.
The fact that there were no warnings and that the attacks were so well synchronised also indicates al-Qaeda could have been involved.
What points to Eta?
Some of the forensic analysis has begun already, and this is mainly what is causing the Spanish authorities to say they believe it is Eta.
They are saying that traces of explosives found match those known to be used by Eta and also match a batch stolen and believed to be in Eta's hands.
However, these attacks would be a quantum leap for Eta. Their largest death toll up to now was in 1987 when they killed 21 shoppers in Barcelona, so this would be in a totally different league for them.
According to the Basque newspaper Gara, Eta has denied responsibility. The group has used the newspaper in the past to issue statements. On Friday, Gara said that someone claiming to represent Eta telephoned on Friday to deny government allegations that it carried out the attacks.
Why would al-Qaeda strike Spain?
The main reasons they would want to hit Spain are in revenge for its assistance to the US and UK in the war on terror and its role in Iraq.
It has had a pro-active role in both of these operations.
The London-based Arabic newspaper al-Quds al-Arabi quoted from a statement in which al-Qaeda reportedly claimed responsibility for the attack.
The statement said the bombs were a punishment for Spain's role in Iraq.
It uses phrases such as "we have got behind the lines" and "we have penetrated the crusader land," which is how the group refers to Europe.
Could this happen in the UK?
The simple answer is yes.
We are not screened in our everyday lives, especially on public transport, and we live in an open democracy.
In a way we are living in a twilight zone in Britain, because it has not happened here yet.