Government and hospital officials could not immediately confirm the latest death from Thursday's simultaneous explosions, the worst such attack in Spain's history.
Nearly 1500 people were also wounded, of whom more than a dozen are described as "critical".
Ten bombs exploded on packed commuter trains entering Madrid on Thursday during the morning rush hour in the worst guerrilla attack in Spain's history. Four others did not go off.
The Basque separatist group, ETA, denied responsibility for the attack.
Meanwhile, a leading newspaper reported on Saturday that the Spanish government told its ambassadors to spread the word that ETA was to blame for the Madrid bombings within hours of the attacks."You should use any opportunity to confirm ETA's responsibility for these brutal attacks, thus helping to dissipate any type of doubt that certain interested parties may want to promote," El Pais quoted Foreign Minister Ana Palacio as writing in a memo.
Officials could not be immediately reached for comment on the report in a paper linked to the opposition Socialists.
Soon after the simultaneous explosions, Prime Minister Jose Maria Aznar's government was quick to point to ETA as its prime suspect, but suspicions have also emerged that Islamist groups may have been behind the attack.
As well as the huge security implications, pinning down responsibility is crucial to Sunday's general election.
Aznar was quick to point to ETA
as government's prime suspect
If ETA is to blame, that could benefit the ruling party because of its tough stance against the Basque separatists.
But if there was al-Qaida or other involvement, it may be viewed as the price of Aznar's support for war in Iraq.
El Pais said Palacio's internal memo, sent at around 5:30 pm (16:30 GMT) on Thursday, quoted earlier statements by Interior Minister Angel Acebes.
"The Interior Minister has confirmed ETA's responsibility. This is confirmed by the explosive and style used, as well as other information that has not yet been made public for obvious reasons," the text said, according to the newspaper.
Since then, the Spanish government has seemed less certain, affirming ETA remains its main line of investigation but saying it is also pursuing other theories.
El Pais said Foreign Ministry officials would not comment on Palacio's memo or whether modified instructions had been sent out to ambassadors at any point afterwards.
"You should use any opportunity to confirm ETA's responsibility for these brutal attacks, thus helping to dissipate any type of doubt that certain interested parties may want to promote"
Spanish Foreign Minister
The newspaper said "an immediate consequence" of Palacio's memo was a "clash" in the UN Security Council between Spain's second most senior diplomat there and the Russian ambassador.
"The latter was reluctant to approve a resolution about the attack in Madrid which condemned ETA, arguing nothing like that had ever been done before, because it is normally impossible to show responsibility for a terrorist act the same day it has been committed," it said.
The UN Security Council voted, however, 15-0 to accept the word of the Spanish government and condemn ETA on Thursday despite hesitations from Russia, Germany and others.