The officials, who requested anonymity, told the newspaper the preliminary investigation and the interrogation of three Moroccans and two Indians arrested in connection with the attack, as well as other evidence, indicated they were carried out by Osama bin Laden's al-Qaeda network.

While Spanish officials have yet to rule out the possibility the Basque separatist group ETA played a role in Thursday's blasts, the Post said some intelligence officials described the pursuit of ETA as a dead end.

Officials told the paper they believed the group that carried out the bombing was composed of Islamic radicals and may have included Saudi nationals and other North Africans besides the arrested Moroccans.