I think this is a good article by BBC:http://news.bbc.co.uk/2/hi/europe/3501364.stm
Madrid blasts: Who is to blame?
In the wake of the deadly attacks on commuter trains in Madrid, it is still unclear who is responsible. The evidence could point to either the armed Basque separatist group Eta or Islamic militants, and the experts are divided. BBC News Online's Robert Plummer analyses the arguments:
THE CASE FOR BLAMING ETA
Spanish police are not closing any lines of inquiry
In the immediate aftermath of the blasts, the Spanish authorities had no hesitation in blaming Eta for the bloodshed.
On Friday, Spanish Foreign Minister Ana Palacio re-stated the government's stance, saying: "Everything appears to indicate that this terrible carnage is the work of Eta."
She acknowledged the possibility that Islamic radicals were behind the attack, saying it was a hypothesis that the government was examining.
However, there were "very strong clues, very strong precedents" to indicate it was the work of the Basque separatist group.
Looking at the evidence that has emerged so far, those clues and precedents include:
the material in the bombs was an explosive used in previous Eta attacks
the attacks were not suicide bombings, but were carried out using remote-controlled detonations - again similar to previous Eta operations
Eta has apparently tried to blow up trains travelling to Madrid before - a previous attack was foiled on Christmas Eve last year and two Eta suspects were arrested
the bombings came three days before the Spanish general election - and over many years, Eta has chosen to stage attacks during election campaigns
If Eta was responsible, analysts say, the simultaneous attacks mark an unprecedented increase in scale, ruthlessness and co-ordination.
Before these bombs, many observers had been tempted to write off Eta as a spent force. Last year, three people died in violent attacks by Basque nationalists - the lowest total for 30 years.
There is no doubt that the Spanish government's uncompromising efforts to crush Eta and refuse all political concessions to Basque nationalism have greatly weakened the separatist group.
Experts are in sharp disagreement over whether Eta in its current state has the capability to mount such a devastating attack.
Eta has the expertise, and it knows how to explode bombs by timers or at a distance, even by mobile phones
Jane's Sentinel Security Assessments
Sebastian Balfour, professor of contemporary Spanish studies at the London School of Economics, told BBC News Online that high-profile arrests of key leaders had reduced the number of Eta activists to no more than 250.
And intelligence analyst Glenmore Trenear-Harvey, interviewed by BBC News 24, said the group would have found it difficult to stage such a co-ordinated effort, given the level of surveillance to which the remaining Eta members are subjected by the Spanish intelligence service.
"To organise something with this number of bombs requires an amazing amount of legwork beforehand," he said.
But Mia Soar, Europe editor for Jane's Sentinel Security Assessments, believes a crackdown by both the Spanish and French governments might have produced a new and more ruthless Eta leadership.
"There have been so many arrests that there has been a constant turn-over of the leadership... they have a lot of young blood at the top. This could explain the new tactics," she told BBC News Online.
Another analyst who blames Eta is University of Kiel security expert Joachim Krause.
The (Spanish) government initially attributed the attack to Eta in good faith. After that there was some confusion
Juan Jose Ibarretxe
"Even though Eta has been partially wiped out, there is still a radical core that has survived and this core apparently has no qualms about killing mass numbers of people," he told Reuters news agency.
However, one leading Basque separatist politician, Arnaldo Otegui, has argued that the apparent lack of a bomb warning in Thursday's attack is not typical of the group.
"Eta has always issued a warning whenever it left a bomb to explode," said Mr Otegui, who formerly led the banned Basque separatist party Batasuna.
And even more mainstream Basque politicians, such as moderate nationalist Juan Jose Ibarretxe, are unsure of Eta's involvement.
Mr Ibarretxe, who is prime minister of the Basque region, said there existed a "reasonable doubt" as to who was behind the attack.
Eta itself has reportedly denied involvement in the attack, through anonymous phonecalls to Basque media used previously for announcements by the separatists.
THE CASE FOR BLAMING AL-QAEDA
The carnage dominated Friday's Spanish press
Few people are inclined at the moment to say categorically that the Madrid bombings were carried out by the Islamic militant network al-Qaeda.
More typical is Richard Evans, editor of Jane's Terrorism and Insurgency Centre, who told Reuters: "It would be premature to rule out at this stage the possible involvement of Islamic militants like al-Qaeda or the wider international jihad."
Evidence for al-Qaeda's responsibility includes:
the choosing of multiple targets in a simultaneous co-ordinated attack is a hallmark of the Islamic militant group
al-Qaeda has threatened revenge on Spain for its government's backing of the US-led invasion and occupation of Iraq
the scale of the devastation dwarfs anything that Eta has done in the past and is much more like an al-Qaeda operation
Spanish police later found a stolen van containing an Islamic tape and seven detonators in the town of Alcala de Henares, where three of the four bombed trains originated
a group called the Abu Hafs al-Masri Brigades has made a claim of responsibility on behalf of al-Qaeda, but it has been widely dismissed
George Joffe, of the Centre for International Studies at Cambridge University, said on the BBC World Service's World Today programme that he had initially believed Eta was responsible, but was now having second thoughts.
"It may yet be that this is part of a much wider conspiracy, and it's certainly true that Spain has in recent weeks arrested large numbers of North Africans, accusing them of being connected with al-Qaeda," he said.
If it is Eta, it's likely that we will know fairly soon. If it's al-Qaeda, we may not get confirmation for months
BBC security correspondent Frank Gardner
The BBC's security correspondent, Frank Gardner, says the UK's Joint Terrorism Analysis Centre, based in the headquarters of the MI5 intelligence service in London, has been analysing the possibility of al-Qaeda involvement in the attack.
Speaking on BBC Radio 4's Today programme, he raised another possibility - that there may have been collusion between an Eta splinter-group and Islamic militants sympathetic to al-Qaeda.
"It would be unusual, because al-Qaeda don't feel comfortable working with non-Muslims, but it's possible that two splinter-groups could decide to pool their resources in this devastating attack," he said.