The bombs were later found to have been detonated by mobile phones. 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 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.
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.