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."

Motivation
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.

Methods
The bombs were believed to have been detonated remotely by combined cell phones and timers, and have been most often described as using an industrially made exlplosive 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.

Goma-2 ECO was used in an April 2004 bomb found, before detonation, along railroad tracks between Barcelona and Madrid.