The investigation: Investigators believe the train bombings that killed 200 people last week were the work of a multinational cell of al-Qaeda loyalists.
Some of these loyalists entered Spain specifically to carry out the attacks and are now the target of an international hunt, according to European and Arab intelligence officials.
The officials said the preliminary investigation and interrogation of five arrested suspects (three Moroccans and two Indian Muslims), as well as other evidence, indicated that the bombings were carried out by al-Qaeda, marking the first time the group has struck in the EU.
For European leaders, the al-Qaeda link is a chilling development and security services across the continent are scrambling to assess the likelihood of further attacks. Analysts said the method of the Madrid attacks - synchronised bombings apparently carried out by remote control rather than by suicide bombers - suggested Islamic extremists had become more adaptable and therefore even more of a threat.
Spanish officials, who initially insisted on attributing the attacks to the Basque separatist group ETA, have contacted security services across Europe and in the Arab world, including Morocco and Saudi Arabia, in an attempt to understand how the plot was put together and executed without even a general warning from intelligence services that Islamic militants were about to strike.
Officials said they believed the group that carried out the bombing was composed of Islamic radicals, possibly including Saudi nationals as well as other North Africans besides the arrested Moroccans. The operation included residents of Spain as well as operatives who entered the country for the attacks, said officials.
Spanish Interior Minister Mr Angel Acebes identified the five detainees at a news conference on Sunday. Investigators said they tied the men to the bomb plot following the discovery of a mobile phone in a gym bag filled with undetonated explosives.
One of the detained Moroccans, 30-year-old Mr Jamal Zougam, had been listed as an al-Qaeda operative in a Spanish judge's 700-page indictment last autumn of Osama bin Laden and others for the terrorist attacks on Washington and New York on September 11th, 2001, officials said. Mr Zougam was not indicted in that case.
Mr Acebes told reporters that the three Moroccans - Mr Zougam, Mr Mohamed Bekkali (31) and Mr Mohamed Chaoui (34) - were known to authorities because of past criminal records in Spain and that at least one might have been involved in a murder. The Moroccan suspects have provided limited information under interrogation, the officials said.
Mr Acebes said authorities had been unable to ascertain the identity of a man shown on a videotape they received on Saturday claiming responsibility for the attacks on behalf of al-Qaeda. The man, speaking in Moroccan-accented Arabic, identified himself as Abu Dujan al Afgani and said he was a spokesman for al-Qaeda's military wing in Europe.
Investigators and analysts said Spain was an inviting target because of President José Maria Aznar's support for the US-led invasion of Iraq and because the country was easily accessible to Moroccans and other North Africans. Spanish officials have not yet ruled out the possibility of indirect ETA involvement in the Madrid bombings, perhaps through the supply of explosives to Islamic militants.
Officials said they were re-examining an alleged sale of explosives by ETA to the radical Palestinian group Hamas several years ago in an effort to identify Basque-Islamist ties. Terrorism experts say Yusuf Galan, a Spanish national who was charged in Madrid in November 2001 with involvement with al-Qaeda, was a former ETA member who had converted to Islam.
Despite these lines of inquiry into a possible ETA role, investigators appear increasingly certain that al-Qaeda was behind last week's attacks, and some intelligence officials have described the pursuit of ETA as a dead end.
Instead, they point to al-Qaeda's use of Spain as a staging ground since the months before the September 11th attacks. The indictment of bin Laden said Spain had served "as a place or base for resting, preparation, indoctrinating, support and financing" terrorist operations.
Mohamed Atta, who led the attacks against New York and the Pentagon, visited Spain twice in 2001, including a 12-day visit in July in which investigators believe he hammered out final details of the plot.
More than 40 al-Qaeda suspects have been arrested in Spain since the September 11th attacks. Although some have been released for lack of evidence, 20 to 30 remain in custody, including several suspected of participation in the Casablanca bombings last year, which killed 45 people.
The prospect of an al-Qaeda campaign in Europe led to meetings across the continent on Sunday, including in Germany, where Chancellor Gerhard Schröder convened his national security council.
Analysts believe the EU will become even more vulnerable in May when 10 new countries join, extending the union's already relatively open borders.
Officials are particularly concerned about an attack on the Olympics in Athens this summer.