As efforts continue to track down those who carried out the 11 March attacks on commuter trains in Madrid, suspicions now appear to focus mainly on Islamic militants, perhaps linked to al-Qaeda. BBC News Online analyses the investigation so far:
- Spanish police identify six Moroccans believed to have carried out the bombings, say Spanish media reports
The bombers targeted four trains
- Five men - three Moroccans and two Indians - are arrested in connection with the blasts on 13 March. They face questioning by a judge who is due to decide whether to charge them
- Five more suspects are arrested on 18 March. Police say four are of Arab origin, one of whom is wanted by Moroccan police for last May's bombing in Casablanca
- An Algerian man, Ali Amrous, is still in custody after being arrested in the Basque city of San Sebastian. He allegedly threatened to cause mass bloodshed in Madrid when questioned by police in January
- A group called the Abu Hafs al-Masri Brigades, linked to al-Qaeda, makes a series of statements claiming responsibility and threatening "America's lackeys" with similar violence
- A video claiming responsibility for the attack purporting to be from al-Qaeda's military spokesman in Europe is uncovered, but its authenticity is yet to be established
- The armed Basque separatist group Eta denies any responsibility
THE CASE AGAINST ISLAMIC GROUPS
Spain's outgoing Interior Minister Angel Acebes believes the inquiry into the Madrid bomb attacks has reached a "decisive phase".
However, investigators still have little to show for their efforts.
Evidence pointing to Islamic militants
Simultaneous attack technique
Iraq war motive
It was the arrests on Saturday of five men that gave the first concrete basis to speculation that Islamic militants were behind the attacks.
The five were detained in connection with a mobile phone which was found inside a bag containing explosives that failed to go off.
Investigators believe mobile phones were used to detonate 10 bombs hidden in backpacks on the four trains which were targeted.
Mr Acebes said all five were believed to be linked to the sale of the phone and falsification of the SIM card. However, none of them has yet been charged.
One of the five, Jamal Zougam, a Moroccan, has been monitored by the Spanish authorities for some time. He is alleged to have links with a man known as Abu Dahdah - identified in a judicial indictment last Autumn as the suspected leader of an al-Qaeda cell in Spain.
Spanish newspapers have reported that Mr Zougam, who ran a mobile phone shop in Madrid, is among six men who police believe carried out the bombings.
Spanish media also suggest that the blasts could be linked to the leading Islamic militant, Abu Musab al-Zarqawi, who is wanted by the United States for a series of attacks in Iraq and elsewhere.
The Moroccan government is sending its own team of investigators to examine a possible connection between the Madrid attacks and a multiple bombing in Casablanca in May last year.
Spain's troop presence in Iraq could have provoked the bombings
That theory gained ground on Thursday, when Spanish police said they had arrested four more people, including one man wanted by Moroccan police for the Casablanca attacks.
Forty-five people died in the Casablanca blasts, which the Moroccan authorities blamed on an ultra-conservative Islamist group said to have links with al-Qaeda.
One of the targets in the attack was a Spanish cultural centre, where four Spaniards were among the dead.
The BBC's security correspondent, Frank Gardner, says Spanish and Moroccan police are working very closely together.
He says that as well as questioning Mr Zougam and attempting to round up the bomb suspects, police are also following up clues discovered by forensic investigators.
He said Spanish media were reporting that the copper detonators used in the bombs had been traced to a quarry in northern Spain.
In the wake of the Madrid bombings, there have been conflicting statements by various Islamic groups.
A man calling himself Abu Dujan al-Afgani and identifying himself as al-Qaeda's military spokesman in Europe has appeared in a videotape claiming responsibility for the attacks.
However, intelligence services have no record of him.
Another group, the Abu Hafs al-Masri Brigades, linked to al-Qaeda, has sent statements to a London-based Arab newspaper saying it carried out the attacks.
The group also warned that its "brigades of death" could strike next in Japan, the US, Italy, Britain, Saudi Arabia or Australia.
Experts are still divided over who is responsible
However, US intelligence officials believe the group has little credibility.
But even before these developments came to light, some analysts were convinced that the bombings bore the hallmarks of al-Qaeda.
The choice of multiple targets in a simultaneous co-ordinated attack is a key feature of an operation by the Islamic militant group.
The attack appears to have been designed to inflict the maximum number of civilian casualties - again consistent with previous al-Qaeda operations.
Al-Qaeda had also threatened revenge on Spain for its government's backing of the US-led invasion of Iraq and its sending of 1,300 troops, which incoming Prime Minister Jose Luis Rodriguez Zapatero has pledged to withdraw.
THE CASE AGAINST ETA
In the immediate aftermath of the Madrid bombing, Spanish officials said the prime suspects were the Basque militant separatist group Eta which has been fighting for an independent Basque state for decades.
The outgoing government of Mr Aznar has consistently refused to negotiate with the group and has outlawed Eta's alleged political wing, Batasuna.
Unlike al-Qaeda, Eta has a history of carrying out attacks to coincide with elections, although it also has traditionally provided warnings if a bomb was about to explode.
Evidence pointing to Eta
History of election attacks
Remote-control bomb technique
Christmas train attack thwarted
The bombs were detonated by remote-control, another Eta hallmark, rather than by suicide bombers, the apparently preferred method of al-Qaeda and Islamic militants linked to it.
It is also thought that the bombs were constructed from the same kind of explosive which has been used in previous Eta attacks.
However, Eta has denied any involvement in the bombings.
Spanish police thwarted an attack last Christmas Eve on a Madrid train station using bombs hidden in backpacks similar to those used in the 11 March attacks. Basque youths were arrested for that plot.
There has been speculation that the Spanish authorities' crackdown could have produced a new and more ruthless Eta leadership or a splinter group.