[4][5][6][7][8][9][10] Controversy regarding the handling and representation of the bombings by the government arose, with Spain's two main political parties—the Spanish Socialist Workers' Party (PSOE) and the Partido Popular (PP)—accusing each other of concealing or distorting evidence for electoral reasons.

[11][4] Immediately after the bombing, leaders of the PP claimed evidence indicating the Basque separatist organization ETA (Euskadi Ta Askatasuna) was responsible for the bombings,[4][12][13] while the opposition claimed that the PP was trying to prevent the public from knowing it had been an Islamist attack, which would be interpreted as the direct result of Spain's involvement in Iraq, an unpopular war which the government had entered without the approval of the Spanish Parliament.

"[15] The prevailing opinion of political analysts is that the Aznar administration lost the general elections as a result of the handling and representation of the terrorist attacks, rather than because of the bombings per se.

[5][7][8] Findings issued by the Spanish judiciary in September 2007 found 21 individuals guilty of participating in the attacks, while rejecting the involvement of an external mastermind or direct al-Qaeda links.

[2] The total number of victims was higher than in any other terrorist attack in Spain, far surpassing the 21 killed and 40 wounded from a 1987 bombing at a Hipercor chain supermarket in Barcelona.

[3] A device composed of 12 kilograms of Goma-2 ECO with a detonator and 136 meters of wire (connected to nothing) was found on the track of a high-speed railway line (AVE) on 2 April.

The suspected militants, Sarhane Abdelmaji "the Tunisian" and Jamal Ahmidan "the Chinese", were trapped inside the apartment by a police raid on the evening of 3 April.

[32] Based on the assumption that the militants killed at Leganés were indeed the individuals responsible for the train bombings, the ensuing investigation focused on how they obtained their estimated 200 kg of explosives.

[39] In December 2004, José Luis Rodríguez Zapatero claimed that the PP government erased all of the computer files related to the Madrid bombings, leaving only the documents on paper.

On 27 May 2005, the Prüm Convention, implementing inter alia the principle of availability which began to be discussed after the Madrid bombings, was signed by Germany, Spain, France, Luxembourg, Netherlands, Austria, and Belgium.

On 4 January 2007, El País reported that Algerian Ouhnane Daoud, who is considered to be the mastermind of the 11-M bombings, has been searching for ways to return to Spain to prepare further attacks,[42] though this has not been confirmed.

[43] On 17 March 2008, Basel Ghalyoun, Mohamed Almallah Dabas, Abdelillah El-Fadual El-Akil and Raúl González Peña, having been found guilty by the Audiencia Nacional, were released after a Higher Court ruling.

[52] The Independent reported that "Those who invented the new kind of rucksack bomb used in the attacks are said to have been taught in training camps in Jalalabad, Afghanistan, under instruction from members of Morocco's radical Islamist Combat Group.

"[5] Mohamed Darif, a professor of political science at Hassan II University in Mohammedia, stated in 2004 that the history of the Moroccan Combat Group is directly tied to the rise of al-Qaeda in Afghanistan.

[56] According to the European Strategic Intelligence and Security Center, the Islamic extremists' alliance with ETA is highly dubious and "there is not anyway any terror case whatsoever to this day in which islamist internationalists collaborated with non-muslims".

[57] Former Spanish Prime Minister José María Aznar said in 2011 that Abdelhakim Belhadj, leader of the Libyan Islamic Fighting Group and current head of the Tripoli Military Council, was suspected of complicity in the bombings.

[58][59] Immediate reactions to the attacks in Madrid were the several press conferences held by the Spanish prime minister José María Aznar involving ETA.

"[62] Due to the government theory, statements issued shortly after the Madrid attacks, including from lehendakari Juan José Ibarretxe identified ETA as the prime suspect, but the group, which usually claims responsibility for its actions, denied any involvement.

[66] Political analysts believe ETA's guilt would have strengthened the PP's chances of being re-elected, as this would have been regarded as the death throes of a terrorist organisation reduced to desperate measures by the strong anti-terrorist policy of the Aznar government.

[12] On the other hand, an Islamic extremist attack would have been perceived as the direct result of Spain's involvement in Iraq, an unpopular war that had not been approved by the Spanish Parliament.

A Renault Kangoo van was found parked outside the station at Alcalá de Henares containing detonators, audio tapes with Qur'anic verses, and cell phones.

[67] The provincial chief of TEDAX (the bomb disposal experts of the Spanish police) declared on 12 July 2004 that damage in the trains could not be caused by dynamite, but by some type of military explosive, like C3 or C4.

[36] Security forces carried out a controlled explosion of a suspicious package found near the Atocha station and subsequently deactivated the two undetonated devices on the Téllez train.

These groups bought the explosives (dynamite Goma-2 ECO) from low-level thieves, police and Guardia Civil informers in Asturias using money from the small-scale drug trafficking.

El Mundo also claimed — among other misgivings[92][93][94] — that the Spanish judiciary reached "scientifically unsound" conclusions about the kind of explosives used in the trains,[95] and that no direct al-Qaeda link was found, thus "debunking the key argument of the official version".

[96] Anthropologist Scott Atran described the Madrid trial as "a complete farce" stating that "There isn't the slightest bit of evidence of any operational relationship with al-Qaida".

Spain's El País newspaper reported that three witnesses testified to seeing him leave a rucksack aboard one of the bombed trains, specifically, the one that exploded at Santa Eugenia station.

This decision for the National Police and the Guardia Civil to strengthen their counter-terrorism services, led to an increase in jobs aimed at preventing and fighting global terrorism.

Sectors of the Partido Popular (PP) and some of the PP-friendly media outlets (primarily El Mundo and the Libertad Digital radio station) claim that there are inconsistencies and contradictions in the Spanish judicial investigation.

It was attended by King Juan Carlos I, Queen Sofía, the victims' families, and representatives from numerous other countries, including British prime minister Tony Blair, French president Jacques Chirac, German chancellor Gerhard Schröder, and U.S. Secretary of State Colin Powell.