How can Europe protect itself from further bomb attacks?
North African extremists are now the main focus of the investigation into Thursday's bombs in the Spanish capital.
Five people have been detained in the last few days in Spain, suspected of involvement in the blasts.
Three of the detainees are Moroccans, some of whom were already being investigated for possible links to Al-Qaeda.
Amongst the five being questioned is Jamal Zougam, a Moroccan, believed to be a follower of al-Qaeda's chief organiser in Spain, who in turn has been linked to the 11 September attacks.
The Spanish are particularly interested in him. Mr Zougam and other suspects have been under surveillance since last May's multiple bombings in Casablanca.
Over 40 people were killed then, and investigators are now probing the links between militant groups in Spain and those in Morocco.
The current line of investigation is leaning towards a group there called Salafia Jihadia.
The Moroccan government have sent their own team to Madrid to investigate some of the people who have been arrested and also to talk about links with Casablanca.
With this in mind, it does seem as if it was unwise of the Spanish government to come out so early and so categorically in favour of blaming Eta.
Counter-terrorism investigations don't move at the speed of 24-hour news. They are quite plodding and methodical, so this hasty judgement - whatever it was based on - has cost the government politically.
Europe now has to wake up to the fact that the organisation we call al-Qaeda is, in fact, a much more complex enemy than we could ever have imagined.
There is a rump leadership of the original al-Qaeda that is hiding out in either Pakistan or Afghanistan - Osama Bin Laden and some of his more familiar deputies and advisers and planners.
But the original al-Qaeda that existed in the camps in Afghanistan in the late '90s is no longer.
It has become a phenomenon - a view of the world which some people aspire to who may not have any actual physical connection with that leadership.
Spain received no warning of the multiple attacks
But the fact that they no longer have a base in Afghanistan means that its ability to plan a really massive attack like 9/11 has definitely been hampered.
It's harder for them to do it because they haven't got the safe home base that they had under the Taleban.
On the other hand it's much harder to detect them now.
There are cells popping up all over the place, many of which the authorities in certain countries don't even know exist yet.
The fact that there was apparently no electronic chatter, that there was no spike in intercepted chatter before this attack in Spain, is causing a lot of people to wonder why they had no warning.