Just months ago, a taped threat thought to be from Usama bin Laden (search) included Spain among countries that could be attacked "at the appropriate time and place."
Bin Laden's warning was contained in an audiotape in October that the U.S. Central Intelligence Agency (search) determined was probably authentic.
Spanish and other anti-terrorism officials say Spain was an important European center for Al Qaeda activity before the Sept. 11, 2001, terror attacks in the United States.
Spanish Judge Baltasar Garzon believes that Spain, along with Germany, was an important staging ground for the hijackings. Lead suicide pilot Mohamed Atta visited Spain twice in 2001, including a trip that July which Garzon says was called to discuss last-minute details with other senior plotters.
Last September, Garzon indicted bin Laden and nine other terror suspects over the Sept. 11 attacks. Three were alleged to be members of a Spain-based terror cell. Garzon charged 25 other men with belonging to Al Qaeda.
More than 40 Al Qaeda suspects have been arrested in Spain since the attacks, although many have been released for lack of evidence. Tayssir Alouni, a reporter for pan-Arab television channel Al-Jazeera who was arrested last September on charges of belonging to Al Qaeda, also has been released on bail.
In Spain, there are fears that Prime Minister Jose Maria Aznar's staunch support of the U.S.-led war in Iraq has made the country a target for Islamic terrorists. Aznar has sent 1,300 troops to Iraq, even though most Spaniards opposed the war.
The first official mention of a possible Islamic angle to Thursday's attacks came when Interior Minister Angel Acebes said that police had found detonators and an Arabic-language audiotape with Quranic verses in a van in a town outside Madrid.
ETA, the separatist group that has claimed responsibility for more than 800 deaths in its decades-long campaign of assassinations and bombings for an independent Basque homeland, remains the "main line of investigation," Acebes said.
But with the van find "all kinds of lines investigation open up," he said. "Because of this, I have just given instructions to the security forces not to rule out any line."
Then, the London-based Arabic newspaper Al-Quds al-Arabi said it had received a claim of responsibility for the Madrid bombings issued by The Brigade of Abu Hafs al-Masri in Al Qaeda's name.
The claim received by e-mail said the brigade's "death squad" had penetrated "one of the pillars of the crusade alliance, Spain," and carried out what it called Operation Death Trains.
"This is part of settling old accounts with Spain, the crusader, and America's ally in its war against Islam," the claim said.
There was no way to verify that the claim did come from Al Qaeda, and Spain's government said ETA remained its No.1 suspect. The 10 bombs on four morning rush-hour trains killed more than 190 people, making it the worst terrorist attack in Spain's history.
Some 500,000 of Spain's 42 million people are Muslims, according to government figures. Neighboring France, in contrast, has an estimated 5 million Muslims.