Spanish officials are investigating conflicting clues following 10 bombings in Madrid that killed at least 190 people and injured more than 1,200.
The blasts hit during the morning rush hour
A statement attributed to al-Qaeda claiming responsibility emerged soon after it was revealed that an Arabic tape had been found in a suspect van.
But other indicators pointed to the Basque armed separatist group Eta.
The interior minister said all lines of inquiry would be followed, as King Juan Carlos tried to comfort the nation.
One claim of responsibility for the rush-hour attacks on trains at three stations was e-mailed to the London-based Arabic newspaper al-Quds al-Arabi.
It said Spain had been targeted by the Abu Hafs al-Masri Brigades on behalf of al-Qaeda as "one of the pillars of the crusade alliance".
"This is part of settling old accounts with Spain, the crusader, and America's ally in its war against Islam," the e-mail said.
A message purportedly from al-Qaeda leader Osama Bin Laden last year threatened Spain and other countries who supported the US-led war in Iraq.
But the BBC's security correspondent, Frank Gardner, says US officials caution that al-Qaeda does not usually claim responsibility so early.
Spanish Interior Minister Angel Acebes said he had instructed security forces not to rule out any line of inquiry after the discovery of a tape in Arabic with detonators in a stolen van near Madrid.
The tape - containing verses of the Koran relating to education - was one of seven in the vehicle found in Alcala de Henares, where three of the four bombed trains originated. The fourth train passed through the town.
But Mr Acebes said the Basque separatist group, Eta, remained the focus of the investigation, echoing earlier statements from officials.
Three days of national mourning has been declared and campaigning before Sunday's general election has been suspended.
Both the king and Prime Minister Jose Maria Aznar gave nationally televised addresses, as they led the grieving for the worst terror attack in Spain's history and the deadliest in Europe since Pan Am flight 103 was blown up over Lockerbie, Scotland, in 1988.
The king said he wished to give all Spaniards an embrace "filled with consolation and sadness".
MAIN ETA ATTACKS
July 2003: Bomb attacks in Alicante and Benidorm, 13 injured. Further explosion at Santander airport days later
Jan, Feb 2000: Car bombs explode in Madrid and the Basque capital Vitoria
June 1998: Car bomb kills Popular Party councillor Manuel Zamarreno
July 1997: ETA kidnaps and kills Basque councillor Miguel Angel Blanco
June 1987: 21 shoppers are killed in an attack on a Barcelona supermarket
1980: In ETA's bloodiest year, 118 people are killed
Dec 1973: Assassination of Prime Minister Admiral Luis Carrero Blanco
But he also struck a defiant note, telling his audience: "Let there be no doubt: terrorism will never achieve its aims.
"It will not manage to break our faith in democracy or our confidence in the future of Spain."
Mr Aznar urged Spaniards to take to the streets to protest against the attacks which he blamed on "the terrorist band" - a term usually used to describe Eta.
"These terrorists wanted to cause as much damage as possible. It is a massacre. They have killed many
people simply for being Spanish."
Security sources said earlier that the bombs set off on the commuter trains used dynamite - typically used by Eta.
The BBC diplomatic correspondent Barnaby Mason says that if Eta was responsible, the near simultaneous attacks carried out with no warning would mark an unprecedented increase in scale, ruthlessness and co-ordination.
The leader of the banned Basque political party Batasuna, Arnaldo Otegi, blamed "Arab resistance".
"Eta has always issued a warning whenever it left a bomb to explode," he said, adding: "Spain maintains occupation forces in Iraq and we should not forget that it had a responsibility for the war in Iraq."
Panic and chaos
The bombs turned the morning commute into carnage when they struck between 0730 and 0800 local time (0630-0700GMT).
The main attack hit one of Madrid's major stations, Atocha, as trains brought thousands of people to work in the capital.
"People started to scream and run, some bumping into each other," Juani Fernandez, a civil servant who was on a platform at Atocha when the bombs went off, told the Associated Press.
Emergency numbers for relatives
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"I saw people with blood pouring from them, people on the ground."
Three other devices hidden in backpacks were defused by police explosives experts.
The attacks were condemned around the world. At least 190 people were killed and more than 1,200 were hurt.
"It was butchery on a brutal scale," said Juan Redondo, an
inspector for Madrid Firefighting Department. "This catastrophe goes beyond the imaginable."
1. Atocha station, 0639 GMT: Four bombs explode on a train approaching the station three blasts on a train inside the station
2. El Pozo station, 0641 GMT: Two explosions
3. Santa Eugenia station, 0642 GMT: One explosion