A year after the attacks, some key details of the plot remain largely unknown. Much about the culprits and their exact motives is still a mystery, and large parts of the investigation, conducted by the Spanish judiciary, are under a news blackout.
The government attributes the attacks to Islamic radicals linked to al Qaeda. The Popular Party -- in power at the time of the attacks -- stands by its initial suggestions that the Basque separatist group ETA was involved. The national daily El Mundo, which has broken several stories on the investigation, has reported extensively on an alleged connection involving elements of the Moroccan secret security services. The Moroccan government denies the allegation.
Names of the victims of the March 11, 2004, train bombings are engraved in a monument in Alcala de Henares, just outside of Madrid. On Friday, the city will mark the year anniversary of the attacks, which killed 191 people.
(Manu Fernandez -- AP)
Photo Gallery: Deadly explosions ripped through commuter trains on March 11, 2004.
A parliamentary commission was set up to investigate the attacks and suggest ways to strengthen security measures. But it deteriorated into a forum for blame-casting between the Popular Party and the Socialists, who won a surprise victory in general elections held three days after the attacks.
Survivors and relatives of victims have been sharply critical of the commission and its report. "They haven't let us know what really happened, and we want to know the truth," said Pilar Manjon, president of the Association of Victims of March 11 Terrorism. She lost her son in one of the bombings.
"The first impression is that it was a wide-reaching plan with multiple explosions perfectly synchronized, backed by important resources and preparation indicating a terrorism of Islamic character," Carlos Divar, president of Spain's highest court, said at a recent news conference. "But I did not say exclusively of Islamic character. We still do not know."
Formal charges are due this summer, with a trial possibly starting early next year.
About 218 people remain hospitalized because of injuries suffered in the attacks.
Correspondent Keith B. Richburg and special correspondent Erika Lorentzsen in Paris contributed to this report.