Islamic extremists have claimed responsibility for the Madrid attack, saying they had acted on behalf of al-Qaeda to avenge the presence of Spanish peacekeeping troops in Iraq.
However, Spanish authorities say they have found no evidence that al-Qaeda ordered or financed the attacks.
Spanish investigators have said one of the fugitive suspects in the Madrid attacks, Moroccan Mohamed Afalah, is believed to have died in a suicide attack in Iraq in 2005.
A confidential police report quoted in Spanish newspapers has said another suspect, Daoud Ouhnane of Algeria, also died in Iraq while fighting coalition forces.
Since the September 11, 2001 attacks in New York and Washington, Spanish police have arrested hundreds of Muslims, many in connection with the Madrid attack.
In recent years police also have focused on cells suspected of collecting money to finance al-Qaeda-linked groups abroad.