In the initial aftermath the Spanish government at the time blame Basque separatist group Eta, despite the scope and scale of the bombings being far greater than the group's usual attacks.

The investigation takes a different turn with the discovery of an abandoned white van containing detonators of a type not used by Eta, and a tape recorded with verses of the Koran at Alcala de Henares station.

Another breakthrough comes when a unexploded bag bomb is found and defused aboard the wrecked train at El Pozo station.

Investigators discover the bombers used mobile phones to set off the devices, and trace the recovered phone's simcard to two Indian salesmen who had sold 13 other identical cards to three Moroccan men - Jamal Zougam, Mohamed Chaoui and Mohamed Bekkali.

Mr Zougam is also identified by survivors of the blasts as a man seen loading bags onto the trains at Alcala de Henares. He is arrested on 13 March and provisionally charged with multiple counts of murder and terrorist offences.

Serial numbers on the outside wrapping of the explosives found in the bag are traced to a mine in northern Spain. On 18 March, Jose Emilio Suárez Trashorras was arrested and charged with supplying some of the explosives to the bombers.

On 16 June, a 16-year-old boy is also arrested and charged with stealing 20kg (44lbs) of explosives. The boy, who cannot be named and is known only by his nickname "El Gitanello" (the little gypsy) later stands trial and is sentenced to six years in a juvenile detention centre.

Fingerprints from an Algerian man, Ouhnane Daoud, are found on the bags recovered at El Pozo. Mr Daoud remains wanted in connection with the attacks.

On 3 April in an effort to trace other phone simcards believed to have been used by the bombers, investigators raid a flat in Leganes, in the suburbs of Madrid.

Before any arrests can be made, seven men inside blow themselves up. Moroccan brothers Mohammed Oulad Akcha, 28, and Rachid Oulad Akcha, 33, are among the dead; police later name them as prime suspects in the train attacks.

Also killed in the flat siege is Serhane ben Abdelmajid Fakhet, also known as "The Tunisian", a man police name as the ringleader of the bombings.

In the months after the attacks, police arrest dozens more suspects in an investigation spanning six countries. To date some 70 people have been arrested.

Investigators believe a network of mainly Morrocan Islamic militants orchestrated the attacks, though members of the former government, now in opposition, have continued to voice the opinion that Eta also had some involvement.

Read more about the continuing investigation
Key suspects