The Madrid attacks fuelled the idea of a 'home-grown' threat
Twenty-eight suspects, mostly Moroccans, are going on trial over the March 2004 Madrid train bombings. The BBC's security correspondent Gordon Corera looks at the significance of the terror attack for Europe.
The explosions, which wreaked carnage on four Madrid trains, left a lasting legacy for Spain, Europe and our wider understanding of the terrorist threat.
The attacks made clear that Western Europe was not immune from the threat of international jihadist terrorism.
After the strike against the US after 9/11, the following two and a half years saw no successful attacks within Western Europe or the US.
As a result, by the start of 2004, the view was beginning to emerge that al-Qaeda might have been significantly disrupted and its ability to strike inside the West severely dented. Madrid showed that this was not true.
More importantly, it showed why this was not true.
The alleged perpetrators may well have been inspired by al-Qaeda's call to re-conquer the lands which were once Islamic, which includes part of Spain.
The attacks were one of the first signs that Europe was vulnerable to groups ... inspired but not formally directed by al-Qaeda
But there are relatively few signs of any direct contact or training from al-Qaeda's leadership and nothing like the kind of evidence which links 9/11 to Bin Laden.
The attacks were one of the first signs that Europe was vulnerable to attacks from groups residing within the continent and who were inspired - but not necessarily formally directed - by al-Qaeda's leadership.
In turn, a new model emerged among counter-terrorism analysts that it was the so-called "home grown" threat that posed the real danger to Europe, rather than jihadists coming into a country to attack it, as happened on 9/11.
But now even this model is being questioned.
Even though the "home-grown" problem of groups inspired by al-Qaeda's ideology remains very much real, there are also signs of a resurgent al-Qaeda leadership, which has managed to regroup on the Pakistan-Afghanistan border and is once again able to communicate to groups and individuals within Europe and pass on instructions.
The Madrid bombings were also important because of the domestic political impact within Spain.
The attack coincided with the closing days of an election campaign and there was a perception that it was intended to inflict damage on a government that had sent troops to fight in Iraq by implying that supporting the US had made the country a target.
Rabei Osman Sayed Ahmed is one of those due to stand trial
The bombings did lead to the withdrawal of Spanish troops from Iraq. But it is sometimes wrongly claimed that the bombings themselves led directly to the defeat of the Conservative government and its replacement just days later by the Socialists.
In fact, it was the perception that the government was misleading the public about who was responsible for the bombings that did most damage.
Government officials indicated they believed the Basque separatist group Eta was responsible, even as evidence emerged to suggest otherwise. This helped galvanise public opposition to the incumbent government.
The legacy of those days has left a lasting bitterness between the two political parties and made the bombings a less "unifying" event domestically than attacks on other countries.
The impact on the election has led experts to speculate that al-Qaeda might well try and repeat the feat by again ordering attacks on the eve of elections to influence their outcome.
But there is little evidence so far for this being the case.
Later in 2004, all al-Qaeda managed was a videotape of Osama Bin Laden to try and influence the US election. But French officials currently appear twitchy ahead of their country's presidential election this spring.
Finally, the Madrid bombings also led to attempts to galvanise Europe's response to terrorism.
Bilateral intelligence sharing increased and the European Union tried to get its act together.
Dutch politician Gijs de Vries was appointed as a co-ordinator in the immediate aftermath of the attacks.
His main role was to try and improve information sharing and co-ordinate and harmonise counter-terrorist legislation and co-operation between member states. He had no formal operational role and a small staff.
Through no want of effort on his part, Mr de Vries found co-ordinating policy among so many countries on such a sensitive issue a tricky task. He has recently announced he will step down in March at the end of his three-year term.
His final months saw controversy over co-operation with inquiries into the CIA practice of extraordinary rendition, in which suspects are picked up by the US, including in Europe, and transported to third countries where it is alleged some may have been mistreated.
The controversy over rendition illustrates how the debate within Europe three years after Madrid has shifted ground and how hard it has been for allies to agree on exactly how to deal with the threat.