Spanish officials continued Friday to hunt for the terrorists behind Madrid's devastating train bombings as one homegrown group initially eyed as the culprit denied responsibility.

"We will bring the guilty to justice," Spanish Prime Minister Jose Maria Aznar (search) said at a news conference. "No line of investigation is going to be ruled out."

As investigators worked on finding who was behind the massive attack that killed nearly 200 people, more than a million Spaniards stood under umbrellas in Madrid in the chilly rain Friday night to protest the bombings.

Interactive: Attacks in Madrid

Many of the demonstrators chanted "Assassins!" and "Cowards!" and shook signs and banners. Earlier Friday, millions of Spanish mourners paid silent tribute to the victims of the attack.

The Basque separatist terror group ETA (search) denied responsibility for the attack on Friday. It was believed to be the first time ETA — Euskadi ta Askatasuna, or Basque Homeland and Freedom — has issued such a denial.

A caller claiming to represent ETA telephoned the pro-Basque daily newspaper Gara and said the separatist group "has no responsibility whatsoever" for the bombings, the paper told The Associated Press. ETA often issues statements through the Basque-language paper.

And Arnaldo Otegi, a top Basque politician, also denied ETA was involved and accused Aznar's government of "lying deliberately" about the bombings to seek political advantage in the upcoming elections. He blamed "Arab resistance" for the attack.

Though Spanish officials initially blamed ETA for the coordinated explosions on packed commuter trains, they were also studying a claim of responsibility by a shadowy group that said it was acting in the name of Al Qaeda (search).

But the Spanish government still leaned toward the theory that ETA was behind the disaster.

"So far, none of the intelligence services or security forces we have contacted have provided reliable information to the effect that it could have been an Islamic terrorist organization," Interior Minister Angel Acebes said Friday.

Ten train bombs were planted during Madrid's morning rush hour Thursday in the worst act of terrorism ever launched on Spain. The attack killed at least 199 people and left more than 1,400 injured, at least three of them Americans, according to the U.S. State Department.

"It just goes to show you that terrorism is a virus that's going to be with us for a very long time," Rep. Robert Andrews, D-N.J., told Fox News on Friday. "We have to work every day on new ways to stop that virus."

The possibility of Al Qaeda involvement arose after police found a stolen van with seven detonators and an Arabic-language tape containing verses of the Quran inside. The van was parked 15 miles outside of Madrid in front of a station where some of the doomed trains originated Thursday.

U.S. Homeland Security Secretary Tom Ridge (search) on Friday wouldn't confirm whether Usama bin Laden's terror ring had a hand in the bombings.

"There is no specific information" available that would point to the identities of the perpetrators, he told reporters during a visit to Thailand. "There is a lot of speculation."

One notion is that an Al Qaeda-linked group contracted out the bombings to ETA (search).

Day of Mourning, Night of Protest

The Madrid blasts occurred exactly 2½ years after the Sept. 11 terrorist attacks in the United States. "March 11, 2004, now holds its place in the history of infamy," Aznar said.

Nationwide rallies called by the Spanish government were held Friday evening. More than a million Spaniards crowded the streets of Madrid for the demonstrations and millions more turned out in Barcelona, Seville, Valencia and other cities — including Spain's Canary Islands off the coast of West Africa.

Nationwide, more than 11 million marched, state TV said.

Aznar and his counterparts from other European countries led one column of the crowd as it snaked down Madrid's main boulevard toward Atocha station, where two of the four bombed trains were attacked Thursday. A stream of people was backed up for miles toward the starting point at Plaza de Colon.

Many of the estimated 2.3 million marchers in Madrid huddled against a steady rain in a bobbing mass of umbrellas that clogged the capital's squares and the area around Atocha.

"It is not raining. Madrid is crying," said Jorge Mendez, a 20-year-old telecommunications student.

Friday night's massive rallies were a remarkable show of unity in a nation divided by regional loyalties and languages.

"We all need to be here to repudiate these killings. All of us. It is our duty," said Manuel Velasco, a university professor who was drenched from the rain.

Marchers held banners reading, "No to Terror" and "Today Our Tears Reach Heaven." Another read simply, "Who and Why?"

"Peace in Madrid and in all of Spain is becoming more remote," said the Rev. Manuel Gonzalez. "We are a passionate people but we want peace."

Earlier Friday during mass silent vigils, people wept openly, prayed and lit candles in the dreary, wet streets of Madrid in the first of three days of mourning — which began at noon with several minutes of silence. Offices, shops and cafes across Spain emptied.

The Spanish capital city on Friday was eerily reminiscent of New York after terrorists hijacked commercial airliners on Sept. 11, 2001, and crashed them into the World Trade Center (search) towers, toppling the buildings and severely damaging several others.

Black bows of mourning dotted Madrid, on shop windows, on flags draped from balconies, and on lapels. Relatives converged on a makeshift morgue, searching for missing loved ones.

Commuters fell silent as their trains rumbled past the bombed-out hulks at Atocha station.

In Barcelona, subways and buses stood still and construction work stopped. In northern Spain's Basque region, hundreds of students and professors at the University of the Basque Country in Leioa stood in silence and clapped afterward.

"This is to show our rejection of violence and our solidarity with the families [of the dead]," said Mikel Luzuriaga, a Basque medical student.

Death Toll Rises

The death toll rose to 199 after hospital officials said a 7-month-old girl died Friday. The baby's mother is apparently hospitalized and her father is missing.

Deputy Justice Minister Rafael Alcala said 84 bodies remained unidentified. Of the more than 1,400 people wounded, 367 people remained hospitalized, about 50 in serious condition.

Aznar said 14 foreigners were among the dead, including three Peruvians, two Hondurans, two Poles and a person each from France, Chile, Cuba, Ecuador, Colombia, Morocco and Guinea-Bissau.

Investigators working through the night took away samples from the twisted wreckage to study the explosives and other data, a senior Spanish police official said.

"They are analyzing absolutely everything," another official said, speaking on condition of anonymity. "All sectors of the police force are involved."

The New York City police department sent two people from the intelligence division to Madrid — a detective expert in bombs and a lieutenant who was assigned to Interpol.

Campaigning was called off for Spain's general election, but Foreign Minister Ana Palacio pledged that the vote would be held on Sunday as planned.

"This would be the first way to tell terrorists about our determination to go forward and that they will not succeed in their wicked cause," Palacio told Australia's Channel Nine television.

The 10 backpack bombs exploded within 15 minutes, starting at about 7:39 a.m., on trains along nine miles of commuter line from Santa Eugenia to the Atocha terminal, a bustling hub for subway, commuter and long-distance trains just south of the famed Prado Museum.

Police later found and detonated three other bombs.

In a chilling account of the bombings, Spanish radio station Cadena Ser broadcast a 12-second recording of an unidentified woman who had called a colleague's voice mail after an initial blast on a train at the Atocha station.

The woman, who survived, was in the process of evacuating as she frantically says: "I'm in Atocha. There's a bomb on the train! We had to —" and then two more blasts are heard amid her screams.

Report: It's Al Qaeda's Fault

The Arabic newspaper Al-Quds al-Arabi said it had received a claim of responsibility issued in the name of Al Qaeda. The e-mail, signed by the shadowy Brigade of Abu Hafs al-Masri (search), was received at the newspaper's London offices and said the brigade's "death squad" had penetrated "one of the pillars of the crusader alliance, Spain."

"This is part of settling old accounts with Spain, the crusader, and America's ally in its war against Islam," the claim said. The claim could not immediately be verified.

The United States believes Al-Masri sometimes falsely claims to be acting on behalf of Al Qaeda. The group took credit for blackouts in the United States and London last year.

"There are so many terrorist groups who are eager to highlight that they can be effective," former FBI agent and Rep. Mike Rogers, R-Mich., told Fox News on Friday. "You need to be careful of any and all groups that initially take credit."

Yigal Carmon of the Mideast Media Research Institute told Fox News he believed the letter claiming to have been written on behalf of Al Qaeda was a fake.

"The language usages ... are not the ones common in Al Qaeda writings," Carmon said. "We are talking here about an 'Al Qaeda group,' which is in fact not a real one."

Spain had backed the U.S.-led war on Iraq despite domestic opposition, and many Al Qaeda-linked terrorists have been captured in Spain or were believed to have operated from there. Aznar sent 1,300 Spanish troops to Iraq.

If the attack was carried out by ETA, it could signal a radical and lethal change of strategy for the group, which has largely targeted police and politicians in small attacks in its decades-long fight for an independent Basque country.

The group usually gives warnings before it attacks and goes after one target at a time, unlike Thursday's hit, which involved multiple strikes and no warning. The largest death toll of an ETA attack was 21 in 1987.

The Spanish government pointed out that ETA had tried a similar attack on Christmas Eve, placing bombs on two trains bound for a Madrid station that was not hit Thursday. The Interior Ministry said tests showed the explosives used in the attacks were a kind of dynamite normally used by ETA.

ETA has claimed responsibility for more than 800 deaths since 1968.

U.N. anti-terrorism chief Inocencio Arias said ETA was likely behind the bombings because they bore "all the fingerprints" of the militant organization.

"I would say it's ETA, but I cannot be sure. It has all the fingerprints of ETA," Arias, a Spaniard who chairs the U.N. Security Council's Counterterrorism Committee, told The Associated Press.

The Madrid train attack came just a week after the French government received threats by a previously unknown terror group saying it would blow up railway tracks there.

Information from the group led to the recovery on Feb. 21 of an explosive device buried in the bed of a railway line near Limoges (search) in central France, the government said. The bomb would have been powerful enough to break a track, it said.

French police said the group, which identified itself as AZF, threatened attacks unless it received $4 million and €1 million — worth $1.2 million — within days. Authorities do not believe the group has any connection to Islamic terror networks.

Fox News' Jonathan Hunt, Catherine Donaldson-Evans and The Associated Press contributed to this report.