Spanish officials initially said it appeared a Basque separatist group was to blame. But later in the day, a London-based Arabic newspaper said it received a letter -- purportedly from Osama bin Laden's al-Qaida network -- claiming responsibility for the train bombings, calling them strikes against "crusaders," Reuters reported.
"This is part of settling old accounts with Spain, the crusader, and America's ally in its war against Islam," said the letter, signed by Abu Hafs al-Masri Brigades. The group has also claimed responsibility for a November bombing of two synagogues in Turkey and the August bombing of the U.N. headquarters in Baghdad.
There was no way of authenticating the letter delivered to the London-based al-Quds al-Arabi newspaper.
Spain was a supporter of the war in Iraq, and an audiotape purportedly of Osama bin Laden warned last year that nations backing the U.S. effort would face retaliation, according to the Association Press.
Spanish police also found a van containing detonators and an Arabic-language tape with Koranic verses near Madrid.
The Basque separatist group initially blamed by Spanish officials, ETA, is fighting for a separate Basque homeland in northwest Spain and southwest France. The issue has been an integral one in Spain's general elections -- just days away.
After the explosions, the government convened an emergency Cabinet meeting and the ruling Popular Party suspended its election campaign, which had focused on a tougher stance against ETA, Reuters reported.
Prior to Thursday, ETA (Euzkadi Ta Askatasuna) had taken responsibility for more than 800 deaths in its decades-old campaign. Those were typically assassinations and smaller-scale attacks, the largest of which was a supermarket blast in Barcelona that killed 21 people in 1987.
Arnold Otegi, leader of Batasuna, an outlawed Basque party linked to the armed separatist group, denied it was behind Thursday's blasts and suggested they were the work of "Arab resistance" elements.
He told Radio Popular in San Sebastian that ETA always phones in warnings before it attacks. The interior ministry said there was no warning before Thursday's attacks, according to the AP.
Despite the claims of responsibility, Interior Minister Angel Acebes told reporters late Thursday that ETA remained a primary suspect, but the investigation would not be limited to the Basque group.
"The conclusion of this morning that pointed to the terrorist organization (ETA) right now is still the main line of investigation. ... (But) I have given the security forces instructions not to rule out anything," Acebes told a news conference.
The series of ten explosions tore gaping holes in rush-hour trains at three stations and scattered bodies among the wreckage.
"The train was cut open like a can of tuna. ... We didn't know who to treat first. There was a lot of blood, a lot of blood," said ambulance driver Enrique Sanchez at Atocha station in central Madrid.
"It is the worst act of terror in the history of Spain and the worst act of terror in memory in any European Union state," European Parliament President Pat Cox told a stunned assembly.
President Bush condemned the bombings as "vicious acts of terrorism."
He called Spain's Prime Minister Jose Maria Aznar to "express his solidarity with the people of Spain at this difficult moment," said White House National Security Council spokesman Sean McCormack.
-- Compiled from wire reports and other media sources