"No line of investigation is going to be ruled out," Spanish Prime Minister Jose Maria Aznar said after a Cabinet meeting. He insisted the probe "will soon bear fruit."
Spanish officials initially blamed Basque separatists for Thursday's attacks but then seemed to backtrack after a van with detonators and an Arabic-language cassette with Quranic verses was found in a stolen van outside Madrid. A shadowy group later claimed responsibility for the bombings in the name of al Qaeda.
The attack occurred exactly two-and-a-half years after the Sept. 11 terrorist attacks in the United States. It also was Europe's worst since the 1988 bombing of a Pan Am jetliner over Lockerbie, Scotland, that killed 270 people.
"March 11, 2004, now holds its place in the history of infamy," Aznar said Thursday.
The 10 backpack bombs exploded in a 15-minute span, starting about 7:39 a.m. on trains along nine miles of commuter line. Police also found and detonated three other bombs.
The death toll rose overnight from 192 to 198, deputy Justice Minister Rafael Alcala said, adding that 84 bodies remain to be identified.
More than 1,400 people were wounded Thursday as panicked commuters trampled on each other, abandoning their bags and shoes. Train cars were turned into twisted wrecks and platforms were strewn with corpses. Cell phones rang unanswered on the bodies of the dead as frantic relatives tried to call them.
"An act of barbaric terrorism has engulfed Spain with profound pain, repulsion and anger," King Juan Carlos said on national television.
As day broke Friday, television and radio re-ran horrific witness accounts of flaming bodies and other carnage on four morning rush hour trains full of workers and students.
Life in Madrid was creeping back to normal, with commuter trains running again except on the line targeted in the attacks, reports CBS News Correspondent Elaine Cobbe.
The government has declared three days of mourning and throughout the capital, flags were at half-mast. Commuters sobbed, lit candles and left flowers at Madrid's Atocha station, which was the first one hit in the attacks, and trains had to roll past wreckage left on the track.
"I saw the trains and I burst into tears. I felt so helpless, felt such anger," said a tearful Isabel Galan, 32, a clerk in a downtown clothes shop.
All the television stations placed a small red and yellow Spanish flag with a black sash in the corner of the screen. Commuter trains also traveled with black cloth on the engine cars.
The government has called nationwide rallies for Friday evening, with Prime Minister Jose Maria Aznar and other senior officials leading one in Madrid. Italian Premier Silvio Berlusconi said he will be there as well. Millions are expected to attend around the country.
Campaigning was called off for Spain's general election, but the vote was still scheduled for Sunday. A major campaign issue was how to deal with ETA, the Basque militant group.
Homeland Security Secretary Tom Ridge said Friday he could not confirm whether al Qaeda 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."
The e-mail claim of responsibility, signed by the shadowy Brigade of Abu Hafs al-Masri and received by the London-based Arabic newspaper Al-Quds al-Arabi, said the brigade's "death squad" had penetrated "one of the pillars of the crusade 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.
Spain's government is studying the claim but still believes ETA is more likely responsible, a senior official in Aznar's office said.
But after police found a stolen van with seven detonators and an Arabic-language tape parked in a suburb near where the stricken trains originated, Interior Minister Angel Acebes said: "I have just given instructions to the security forces not to rule out any line of investigation."
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.
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 here.
Just months ago, a taped threat thought to be from Osama bin Laden included Spain among countries that could be attacked "at the appropriate time and place."
A top Basque politician, Arnold Otegi, denied ETA was behind the blasts and blamed "Arab resistance," noting Spain's support for the Iraq war.
If the attack was carried out by ETA, it could signal a radical and lethal change of strategy for the group that has largely targeted police and politicians in its decades-long fight for a separate Basque homeland.
The government said 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 Thursday's attacks were a kind of dynamite normally used by ETA. But Thursday's violence consisted of multiple attacks and no warning.
ETA has usually gone after one target at a time and the largest casualty toll was 21 killed in 1987. ETA has claimed responsibility for more than 800 deaths since 1968.
The United States, Britain and Russia said the attacks demonstrated the need for toughened resolve against terrorists.
President Bush called Aznar and Juan Carlos, saying he expressed "our country's deepest sympathies toward those who lost their life."