The attacks, the deadliest against civilians on European soil since the 1988 Lockerbie airplane bombing, were initially suspected to be the work of the Basque separatist militant group ETA. This was soon proved incorrect as evidence mounted against an extreme Islamist militant group loosely tied to, but thought to be working in the name of, al-Qaida.
Investigators believe that all of the blasts were caused by improvised explosive devices that were packed in backpacks and brought aboard the trains. The terrorists seem to have targeted Madrid's Atocha Station, at or near which seven of the bombs were detonated. The other bombs were detonated aboard trains near the El Poso del Tio Raimundo and Santa Eugenia stations, most likely because of delays in the trains' journeys on their way to Atocha. Three other bombs did not detonate as planned and were later found intact.
Many in Spain and around the world saw the attacks as retaliation for Spain's participation in the war in Iraq, where about 1,400 Spanish soldiers were stationed at the time. The attacks took place two days before a major Spanish election, in which anti-war Socialists swept to power. The new government, led by Prime Minister Jose Luis Rodriguez Zapatero, removed Spanish troops from Iraq, with the last leaving the country in May 2004.
A second bombing, of a track of the high-speed AVE train, was attempted on April 2, but was unsuccessful. The next day, Spanish police linked the occupants of an apartment in Leganes, south of Madrid, to the attacks. In the ensuing raid, seven suspects killed themselves and one Spanish special forces agent by setting off bombs in the apartment to avoid capture by the authorities. One other bomber is believed to have been killed in the train bombings and 29 were arrested. After a five-month-long trial in 2007, 21 people were convicted, although five of them, including Rabei Osman, the alleged ringleader, were later acquitted.
In memory of the victims of the March 11 bombings, a memorial forest of olive and cypress trees was planted at the El Retiro park in Madrid, near the Atocha railway station.