Suri, also called Mustafa Setmarian Nasar, is thought to be 45 years old. European intelligence agencies have said that he was once the overall commander of al Qaeda training camps in Afghanistan and once headed al Qaeda's propaganda operation.
After the attacks of Sept. 11, 2001, investigators thought he was part of an al Qaeda faction that distanced itself from bin Laden's leadership. But more recently, European intelligence agencies have questioned that view and come to believe Suri may have traveled to Europe last year to activate some of the al Qaeda groups in Spain and elsewhere.
A closed-circuit television image shows people fleeing moments after a bomb exploded at a Madrid train station in March.
The investigations into the attacks, including one by a parliamentary commission that has often degenerated into partisan bickering, have also suggested that Spanish authorities, who for years have been focused on blocking Basque terror attacks, may have missed key chances to discover the March 11 plot.
Many of the key suspects had been under surveillance for almost a year before the train attacks, including Fakhet, one of the eight men who died in the apartment. But the surveillance by Madrid's anti-terrorist unit ended in February 2004, the month before the bombings.
Officials of the unit, in documents and testimony, have told the parliamentary inquiry that they stopped their surveillance because of a lack of manpower and a need to shift their focus to a possible ETA attack before the elections and to security for May's wedding of the crown prince.
Other reports handed over to the parliamentary commission reveal how Spain's national police warned as early as the summer of 2003, after a series of simultaneous bomb attacks in Casablanca, that Spain was at increased risk of attacks from Islamic radicals.
Some people who have closely followed the investigations suggest that the bombings were not specifically timed for the election.
In other intercepted telephone conversations reported by El Mundo and broadly confirmed by Spanish authorities as accurate, Ahmed was overheard saying the planning for the March 11 attacks took 2 1/2 years. That would mean it was in motion well before the election was called, and trying to influence the vote became a secondary objective, if an objective at all.
Some experts believe the elections only provided the attackers a chance to make a bigger impact, but they did not necessarily set out to change the result of the vote. The experts also said that because the police had shifted their focus to the ETA for the elections, the date may have been chosen because it afforded a better chance to execute the plot undetected.
The police "were not looking for Arabs -- they were looking for ETA," said Miguel Angel Bastenier, foreign policy editor at El Pais newspaper. The attackers "took advantage of the fact that there was an election three days later to magnify their attack."
"The terrorists were quite cognizant of the fact that they could piggyback on the publicity of an election," said Bruce Hoffman, a terrorism analyst who heads the Washington office of the Rand Corp. "I don't think they had any idea they could affect the outcome. I don't think they could have orchestrated such a favorable constellation."
Staff writers Dan Eggen and Dana Priest and staff researcher Robert E. Thomason in Washington and special correspondents Pamela Rolfe in Madrid and Sarah Delaney in Rome contributed to this report.