At least 190 people were killed and 1,240 wounded Thursday as 10 bombs rocked three Madrid train stations during the height of the morning rush hour. The attack took place just three days before Spain's general elections.
A van containing several detonators and an Arabic-language tape of Koranic verses had been found near Madrid, Interior Minister Angel Acebes said later Thursday, announcing that new lines of investigation into the bombings were being opened.
Until that point, suspicion had focused on Spain's primary domestic terrorists, the Basque separatist group ETA (search).
"In this moment of pain, all Spaniards are called more than ever to end terrorism and violence," Spanish King Juan Carlos (search) said during a televised address to the nation. "Let there be no doubt: Terrorism will never prevail."
After an emergency Cabinet meeting, a somber Prime Minister Jose Maria Aznar (search) called the attacks "mass murder" and vowed to hunt down the attackers. He reaffirmed his policy of not negotiating with ETA.
"No negotiation is possible or desirable with these assassins who so many times have sown death all around Spain," Aznar said.
Police were looking for at least two people seen jumping on and off one of the trains at a station in Madrid.
The bombers used titadine (search), a kind of compressed dynamite, a source at Aznar's office said, speaking on condition of anonymity.
On Feb. 29, titadine was among the explosives found packed into a van that had been pulled over outside Madrid. Two alleged ETA members were arrested, but their identities were withheld.
The February plan was to "generate a massacre in coming days, if possible, in the center of Madrid," Justice Minister Jose Maria Michavila said then.
Francisco Javier Ruperez, Spain's ambassador to the United States, told Fox News he had "no doubt" that ETA was behind Thursday's attacks.
Ruperez, who was kidnapped by ETA in 1979, admitted there was "no smoking gun" linking Al Qaeda with ETA, but added that "at the end of the day," terrorist organizations "tend to share the same sympathies ... the same aims."
A U.S. intelligence official, speaking on condition of anonymity, said "it's too early to tell" who was behind the Madrid blasts, adding that "we're not ruling anything out."
Mansoor Ijaz, a foreign-affairs analyst for Fox News, said the attacks had many of the hallmarks of an Al Qaeda operation. He said it was evidence the pan-Islamic terror organization may be "joining hands with local terrorists."
"This represents a dangerous mutated version of what Al Qaeda has been doing in other parts of the world," Ijaz explained, "hitting three simultaneous targets, not necessarily in the same city but in the same area, with multiple explosions at each location."
Ijaz said Madrid was part of "an emerging pattern," citing recent multiple bombings in Iraq that may have been Al Qaeda-inspired. He noted that Spain has been a staunch supporter of U.S.-led military efforts in Iraq.
Arnold Otegi, leader of Batasuna (search), an outlawed Basque political party linked to ETA, denied the domestic terror group was behind the blasts and suggested "Arab resistance" elements were instead responsible.
Otegi told Radio Popular in San Sebastian that ETA always phoned in warnings before it attacked. Spain's interior minister said there had been no warning before Thursday's explosions.
"The modus operandi, the high number of victims and the way it was carried out make me think ... it may have been an operative cell from the Arab resistance," Otegi said.
'I've Seen Horror'
"We are living in difficult times but we're happy to see that anonymous individuals are offering their services, helping to transport individuals, helping to donate blood and acting with great dignity," Aznar said in his address to the country around 9 a.m. EST.
President Bush called Aznar to express solidarity with the Spanish people and to offer his condolences, condemning "this vicious attack of terrorism in the strongest possible terms," National Security Council spokesman Sean McCormack said.
Secretary of State Colin Powell said he spoke with his Spanish counterpart and offered sympathy and solidarity in the war on terrorism.
"The United States stands resolutely with Spain in the fight against terrorism in all its forms and against the particular threat that Spain faces from the evil of ETA terrorism," Powell said.
A total of 10 bombs, nearly all in backpacks, exploded in a 15-minute span along nine miles of the commuter line -- running from Santa Eugenia to the Madrid hub of Atocha -- killing 190 people and injuring more than 1,240, Interior Minister Angel Acebes said.
Police found and detonated three other bombs.
The blasts began about 7:40 a.m., tearing through trains or platforms on the commuter line running to the Atocha station. At least two of the bombs went off in trains at that station.
Worst hit was a double-decker train at the El Pozo station, where two bombs killed 70 people, fire department inspector Juan Redondo said. El Pozo is about six miles from Atocha.
People in tears streamed away from the station as rescue workers carried bodies covered in sheets of gold fabric. People with bloodied faces sat on curbs, using cell phones to tell loved ones they were alive. Hospitals appealed for blood donations. Buses had to be pressed into service as ambulances.
Rescue workers were overwhelmed, said ambulance driver Enrique Sanchez.
"There was one carriage totally blown apart," he said. "People were scattered all over the platforms. I saw legs and arms. I won't forget this ever. I've seen horror."
Shards of twisted metal were scattered by rails in the Atocha station at the spot where an explosion severed a train in two.
"I saw many things explode in the air, I don't know, it was horrible," said Juani Fernandez, 50, a civil servant who was on the platform waiting to go to work.
British Foreign Secretary Jack Straw (search) called the attacks terrorist atrocities and a "disgusting assault on the very principle of European democracy."
European Parliament President Pat Cox (search) said the bomb attacks amounted to "a declaration of war on democracy."
"No more bombs, no more dead," Cox said in Spanish before a hushed legislature in Strasbourg, France. "It is an outrageous, unjustified and unjustifiable attack on the Spanish people and Spanish democracy."
More than eight in 10 Spaniards said in an Associated Press-Ipsos poll taken last month that they are worried about the threat of terrorism in their country. That was the highest level of concern about terrorism in five European countries polled — Britain, France, Germany, Italy and Spain.
ETA had been blamed for more than 800 deaths in its decades-old campaign to carve an independent Basque homeland out of territory straddling northern Spain and southwest France.
Until now, the highest death toll in an ETA attack was 21 killed in a supermarket blast in Barcelona in 1987.
The Associated Press contributed to this report.