2004 Madrid bombings
On 11 March 2004, in Madrid, 191 people were killed and over 1800 injured by a series of ten terrorist bombs in four commuter trains. While there was initial suspicion of Basque separatists, it is now believed that the act was carried out by Islamist radicals.
Multiple simultaneous attacks have been a signature of al-Qaeda, but the attack does not appear to have been under the operational control of "al-Qaeda Central."
According to Fernando Reinares, participants in the bombings were first thought to been self-radicalized, but, according to Reinares, now appears that most were radicalized before the 9-11 Attacks and the Iraq War, but the actual group coalesced in Spain. Most were economic immigrants although one was a native Spaniard. Internet access complemented face-to-face group formation, allowing downloading of such things as the works of Sayyid Qutb and Abdullah Azzam.  Indeed, "a gradual sense of progressive involvement, usually considered a consistent quality among those becoming terrorists, seems to be absent for some latecomers in the Madrid bombing network. 
Spanish internal and international politics were a factor. Initially, the government of Prime Minister of Spain Jose Maria Aznar blamed the Basque organization, ETA, as "public enemy number one." Aznar's government also had sent troops to the Iraq War, a decision much criticized by the mainstream political opposition. Since opposition leader Jose Ruiz Rodriguez had promised to withdraw the troops if elected, this produced an opportunity for solidarity among Islamic radicals. 
13 bombs were placed, although only 10 detonated. They were controlled by combined cell phones and timers, and have been most often described as using an industrially made explosive called Goma-2 ECO, supplemented with metal fragments to maximize casualties. There have been some disputes about the nature of the explosive, and it has been noted that Basque bombings typically used a different material. At a trial in April 2007, prosecutors said approximately 200 kilograms came from "Emilio Trashorras, a Spanish miner accused of stealing it from the Asturian mine where he formerly worked and exchanging it for hashish with Rafa Zouhier, a drug trafficker. The deal was allegedly struck in a roadside McDonald's near the northern town of Tineo, and the explosive then brought to Madrid in a truck."
Investigation and trial
During the active investigation, an initial three suspects, later corrected to seven, detonated suicide bombs at their hideout in Leganes, Spain a suburb south of Madrid. These included Sarhane Ben Abdelmajid. One police officer was killed and 11 wounded. 
The accused leader leader and other 29 survivors were brought to trial in 2007. Rabei Osman Sayed Ahmed, known as "Mohamed the Egyptian", denied all connection. Key evidence, however, comes from a wiretap that Ahmed denies has his voice:
The entire operation of Madrid was my idea...They were my dearest friends ... they were martyrs for whom Allah have mercy ... The thread of the operation of Madrid was mine. Understand?... The trains ... they were all my group. In reality, I was not with them the day of the operation but on 4 March I was in contact with them and was abreast of all the details.
The BBC reported that prosecutors said Mr Osman had lived, in Madrid a Tunisian, Sarhane Ben Abdelmajid. Osman was linked to Youssef Belhadj, who is said to be al-Qa'ida's military leader in Europe, and Hassan el Haski, accused of heading the Spanish branch of the Moroccan Islamic Combat Group.
- Fernando Reinares (November 2009), "Jihadist Radicalization and the 2004 Madrid Bombing Network", CTC Sentinel, Combating Terrorism Center, United States Military Academy 2 (11), pp. 16-19
- John Horgan, “From Profiles to Pathways and Roots to Routes: Perspectives from Psychology on Radicalization into Terrorism,” Annals of the American Academy of Political and Social Sciences 618:1 (2008): pp. 80-93, cited by Reinares 2009
- Teemo Sinkkonen (27 November 2009), Political Responses to Terrorism, University of Tampere, dissertation, p. 12
- Elizabeth Nash (16 February 2007), "'Mastermind' of Madrid bombing goes on trial", Independent (UK)
- Rail line bomb 'matches Madrid', BBC News, 3 April 2004
- "Madrid bombing suspects kill themselves", CBC News, 4 April 2004