Spain has detained and provisionally charged several suspects in connection with the 11 March train bombings in Madrid, many of them Moroccan.
Police have pieced together clues from the unexploded bombs
Four suspects died during a police raid on 3 April, including the alleged ringleader, Serhane ben Abdelmajid Fakhet, alias "The Tunisian", the interior ministry said.
BBC News website tracks the key developments in the investigation:
11 March: Ten bombs explode on four packed early-morning commuter trains in Madrid, killing 191 people and leaving at least 1,800 injured.
Police also carry out controlled explosions on three other unexploded devices, which were hidden in rucksacks.
In the immediate aftermath of the attacks, the Spanish government names the Basque separatist group Eta as the main suspect. But evidence simultaneously emerges that Islamic militants might be behind the attacks.
The evidence includes the discovery of a stolen van containing seven detonators and an Arabic language tape close to a Madrid station.
A letter sent to a London-based Arabic newspaper claims responsibility for the attacks on behalf of the Abu Hafs al-Masri Brigades, a group that aligns itself to
Osama Bin Laden's al-Qaeda network. The accuracy of the claim cannot be verified.
13 March: Five men are arrested in connection with a mobile phone found inside a bag of explosives which failed to go off. They are three Moroccans and two Indians. One of the Moroccans is named as Jamal Zougam, who is alleged to have links with the suspected leader of an al-Qaeda cell in Spain.
14 March: A video claiming responsibility for the attack purporting to be from al-Qaeda's military spokesman in Europe is uncovered. The speaker says the attacks were revenge for Spain's "collaboration with the criminals Bush and his allies".
Spain's Socialist Party wins a surprise victory in the country's general election, after voters appear to turn on the government over its handling of the Madrid attacks.
15 March: The man set to be the new prime minister, Jose Luis Rodriguez Zapatero, condemns the war in Iraq and threatens to pull Spanish troops out of the country.
18 March: Five men are arrested over the Madrid attacks - four Moroccans and a Spaniard. One of the Moroccans is freed without charge a few days later.
19 March: The five men arrested two days after the attacks are remanded in custody. They are:
Moroccans Jamal Zougam, Mohamed Bekkali and Mohamed Chaoui, who are provisionally charged with multiple counts of murder, attempted murder, stealing a vehicle, belonging to a terrorist organisation and four counts of carrying out terrorist acts.
- Two Indian nationals Vinay Kohly and Suresh Kumar, held on suspicion of collaborating with a terrorist organisation, of fraud and of falsifying documents.
20-21 March: More suspects arrested in separate raids. They include a two Moroccan men and a Moroccan woman.
23 March: Four men arrested on 18 March are remanded in custody. They are:
- Spaniard Jose Emilio Suarez Trashorras, accused of supplying the explosives for the attacks and facing charges of multiple counts of murder, as well as attempted murder, robbery and terrorism charges.
- Moroccan Abderrahim Zbakh, accused of multiple murders and attempted murders, robbery and collaborating or belonging to a terrorist organisation.
- Moroccans Mohamed El Hadi Chedadi and Abdelouahid Berrak, facing charges of collaborating with or belonging to a terrorist organisation.
Another Moroccan man who was being held is freed without charge.
The same day, another Moroccan man is arrested over the Madrid blasts.
24 March: Syrian man arrested.
Two more suspects are provisionally charged with collaborating with a terrorist group - a Moroccan man, Rafa Zuher, and the only woman suspect so far, Naima Oulad Akcha, also Moroccan.
26 March: Another suspect, Moroccan Faisal Alluch is provisionally charged with collaborating with a terrorist group.
30 March: Spain names an Islamist extremist group, the Moroccan Islamic Combatant Group, as the main focus of the Madrid investigation.
Provisional charges are laid against two more suspects held in detention. They are:
Syrian Basel Ghayoun, accused of mass murder and belonging to a terrorist organisation.
- Moroccan Hamid Ahmidam, accused of collaborating with a terrorist organisation.
Spanish police also arrest a Moroccan man, Otman El Gnaout.
31 March: Spain issues international arrest warrants for five Moroccans and a Tunisian, identified as Sarhane ben Abdelmajid Fakhet and described as the leader of the group suspected of carrying out the Madrid attacks.
2 April: An unexploded bomb is found on the high-speed rail link between Madrid and Seville. Tests later show that the explosives matched the type used in the 11 March attacks.
Provisional charges are laid against a 15th suspect. Moroccan man Otman El Gnaout is accused of collaboration with or membership of a terrorist organisation.
3 April: Four suspects wanted for the Madrid train bomb attacks die during a police raid on an apartment in Leganes, a suburb of Madrid, the interior ministry says.
Among the dead is the alleged ringleader of the Madrid bombings, Sarhane ben Abdelmajid Fakhet, a Tunisian named in the international arrest warrants, the authorities say.
Another of those killed is identified as Abdennabi Kounjaa, a Moroccan also named in the arrest warrants, Spain says.
A third man is named as Arrisse Rifat Anouar.
One police officer is killed and at least 11 injured.
Officials announce two more arrests - one in Madrid, the other in Ceuta.
Police examine a fax threatening to "make blood flow like rivers" if Spanish troops remain in Iraq and Afghanistan. The fax comes from a group claiming both the 11 March bombings and the foiled attack on the Madrid-Seville rail link.
Two Moroccans are detained and provisionally charged with terrorism offences. Rachid Adli is arrested near Madrid, and is reported to have admitted meeting Jamal Zougam, one of the key suspects in custody.
The other Moroccan, Abdelilah el Fuad, is arrested in the Spanish enclave of Ceuta in North Africa. Spanish media said he had been at a house where the bombers are believed to have prepared the devices and activated the mobile phones that triggered the train explosions. Mr Fuad is also said to have bought a car used to go to Asturias to collect explosives for the 11 March attacks.
Three more suspects arrested: Fouad el Mourabit, a Moroccan, is provisionally charged with collaborating with terrorists. The other two suspects are not named.
Spanish police arrest three more suspects, reportedly from Morocco, Saudi Arabia and Egypt. Their names are not released.
The interior ministry issues five new arrest warrants - and the pictures of the suspects wanted in connection with the attacks.
Their names are: Mohamed Afalah, Mohamed Belhadj, Abdelmajid Bouchar, Mohamed Bouharrat and Hicham Ahmidan. Their nationalities were not given.
Earlier in the day, a Spanish magistrate investigating the bombings ordered two Moroccan brothers, Abdennabid and Mohamed Cheddadi, to be freed.
But he ordered the brothers to keep the court informed of their whereabouts, a court official said.