Through anonymous phone calls to Basque media outlets, ETA vehemently denied involvement.
Islamic militants who were based in Spain but inspired by al Qaeda were designated later as the prime suspects.
March 11, 2004 - Coordinated attacks including 10 bombs on four commuter trains at three different stations kill 191 people and wound more than 1,800.
March 13, 2004 - An al Qaeda claim of responsibility is made via video tape by a man speaking in Arabic with a Moroccan accent.
March 13, 2004 - Five people are arrested in connection to the case 60 hours after the bombings. Three of those arrested are Moroccans, and two are Indian. Prepaid phone cards and a cell phone from backpacks found at the bombing site link the five to the investigation.
March 14, 2004 - The Spanish Interior Ministry releases the names of five people detained in connection with the attacks. The men are identified as Jamal Zougam, Mohamed Bekkali, Mohamed Cahoui, Vinay Kohly and Sureh Komar.
March 18, 2004 - Spanish authorities arrest four North Africans in connection with the bombings. The radio report says three were arrested in the Madrid suburb of Alcala de Henares and the other North African was arrested in northern Spain. They are: Abderrahim Zbakh, Farid Oulad Ali and Mohamed El Hadi Chedadi, whose brother, Said Chedadi, was indicted last September by a Spanish judge for links to al Qaeda.
- The fourth suspect is not identified but is described as being of Arab descent.
- The fifth suspect is a Spanish citizen who goes by the name of Jose Emilio Suarez Trashorras. He is arrested in northern Spain.
March 19, 2004 - Spain's National Court charges five suspects in connection with the bombings and remands them into custody after an all-night court session. The Court also releases Ali Amrous, an Algerian man held in connection with the Madrid terror attacks and suspected of being an al Qaeda member.
March 22, 2004 - Spanish state radio reports four new arrests in the Madrid bombings.
March 24, 2004 - A Spanish judge charges two more suspects, Naima Oulad and Rafa Zouhier, in the train bombings, bringing the total number of people charged in the attacks to 11.
March 25, 2004 - A Spanish judge charges a Moroccan man, Faisal Alluch, with collaborating with a terrorist group in connection with the train bombings, boosting the number to 12 suspects who have been charged in the case.
March 30, 2004 - Spanish Interior Minister Angel Acebes names a Moroccan terrorist group, Moroccan Islamist Combat Group (GICM), as the principal focus in the investigation.
March 30, 2004 - Moroccan Fouad El Morabit, who had been released without charges, is re-arrested. Court sources also confirm the latest arrest in the case, a man identified as Otman el Gnaout.
March 30, 2004 - Basel Ghayoun, a Syrian man, is charged in the bombings. Hamid Ahmidan of Morocco is charged with collaborating with a terrorist group and a count of drug possession. Three other men are released.
March 31, 2004 - A Spanish National Court judge issues international arrest warrants for six more suspects as the investigation focuses on the GICM. The Interior Ministry says five of the men sought are Moroccans. They include two brothers and a man who is related to other Moroccans previously arrested. The sixth man sought is Tunisian.
March 31, 2004 - Arraignments begin for two men, Antonio Toro Castro of Spain and Mustafa Ahmidam from Morocco.
April 2, 2004 - A bomb found under high-speed rail tracks between Madrid and Seville appears to be made of the same explosives used in the March 11 attacks.
April 2, 2004 - A Spanish judge releases without charges two Syrian men who had been detained in connection with the March 11 Madrid train bombings. He also frees a Moroccan man but orders him to report daily to police until further notice.
April 3, 2004 - Seven suspected terrorists kill themselves and a policeman when they set off an explosion in a suburb of Madrid as police attempt to enter a building. The suspects are presumed to be involved in the train bombings. Fingerprints at the scene later result in more arrests, including Saswan Sabagh.
April 3, 2004 - Spanish authorities arrest two more people but the identities of the two are not released.
April 7, 2004 - A National Court judge charges two more Moroccan suspects, Abdelilah El Fuad and Rachid Adli, in the March 11 Madrid train bombings.
April 12, 2004 - Spanish police arrest three more suspects. One of the three was identified as Morabit, who has now been detained three times. The other two are not identified.
May 6, 2004 - Brandon Mayfield, an American attorney, is taken into custody by the FBI in connection with the attacks. His fingerprints were found on a bag containing detonators of the kind used in the attacks, in close proximity to the blast site. The Spanish Interior Ministry spokesman said the plastic bag was found inside a stolen van left near the Alcala train station, from which the three bombed trains departed. US sources are calling him a material witness, not formally charging him with a crime as of yet, and state that he is a follower of Islam.
November 2004 - Spanish lawmakers launch an inquiry into the train bombings.
January 2005 - Spain's interior minister says Spanish officials have made 66 arrests in the train bombing investigation.
April 11, 2006 - Twenty-nine people are indicted in a Spanish court in connection with the bombings. Five men are charged with planning and carrying out the plot, and a sixth is named as a "necessary collaborator." The rest are charged with supporting roles.
February 15, 2007 -
Start date of trial for 29 defendants.
Seven defendants are considered prime suspects, and they each could face sentences of about 38,000 years in prison for mass murder, if convicted.
June 4, 2007 -
One of the 29 defendants in the Madrid train bombings trial, Brahim Moussaten, has been cleared of all charges and is now a free man,
a court spokeswoman tells CNN.
October 31, 2007 - Verdicts are read for the remaining 28 defendants. Three men are found guilty of the most serious charges and sentenced to thousands of years in prison. However, under Spanish law, they will serve only 40 years. Eighteen defendants are found guilty of lesser charges. Seven defendants are acquitted, including alleged mastermind Rabei Osman.
July 17, 2008 - Four defendants, Basel Ghalyoun, Mouhannad Almallah Dabas, Abdelilah el-Fadual al-Akil and Raúl González, have their convictions overturned. The acquittal of Osman is also upheld.
December 18, 2008 - A criminal court in Morocco convicts Abdelilah Ahriz of belonging to a terrorist group involved in the train bombings and sentences him to 20 years in prison. Prosecutors originally requested that Ahriz be given a life sentence, saying DNA sampling proved his involvement in preparing the train bombings.
May 12, 2009 - Ten of the 14 suspected Islamic militants accused of assisting the three suspects are acquitted by Spain's anti-terrorism court. The ruling gives the remaining four sentences between two and nine years for falsifying documents or being part of a terrorist group.
January 13, 2010 - A Spanish court convicts five men accused of Islamic terrorist activities, including aiding fugitives from the Madrid train bombings of 2004 and planning other attacks. Their sentences, on charges of collaborating or belonging to an Islamic terrorist group, range from five to nine years in prison.
February 2011 - Spain's Supreme Court overturns the lower court's conviction of the five men convicted in January 2010 for Islamic terrorist activities that included aiding fugitives from the Madrid train bombings and planning other attacks.