The first blast at the height of the morning rush hour ripped apart a train packed with commuters, just as it approached Madrid's main railway station. Minutes later, more explosions and more carnage at two suburban rail stations.

In addition to the Madrid-bound truck packed with 500 kilograms of explosives late last month, in December, police thwarted a planned bombing at another Madrid rail station, arresting two suspected ETA members.

ETA has been waging a war for more than 30 years against the Spanish state to establish an independent Basque homeland in northern Spain and southern France. But most experts say it can, at best, count on the support of no more than 10-15 percent of the Basque people.

Gustavo de Aristegui, a Basque member of parliament from the ruling conservative Popular Party, says ETA represents a tiny minority that is trying to impose its will on Basques and other Spaniards.

"There are people who are real monsters, who are trying to blackmail a whole society through acts of terror. And what we have to do is to keep united and build a political consensus to defeat terrorism," he said.

But a leader of Batasuna, a banned political party that is sympathetic to ETA, denied the group's involvement. Arnold Otegi says the explosions were the work of what he called the Arab resistance, a reference to Spain's support of the U.S.-led war in Iraq and the fight against terrorism.

That led to a furious response from Spanish Interior Minister Angel Acebes.

"Mr. Otegi has given disinformation to detract attention from the real culprits. This way, it would create confusion, and it would multiply the fear and achieve what the terrorists are aiming at - to create fear and confusion," he said. "And I think it is a strategy, a miserable strategy like everything else ETA does, and everyone that supports them. But I don't think that, at present, anybody has any doubts."

A security analyst at the University of London, Dan Plesch, says if the attacks are the work of ETA, they represent a new benchmark for a group that has usually targeted individual officials and police officers.

"ETA has always displayed an ability to have very professionally organized, unfortunately, terrorist guerilla operations," he noted. "And seeing this seems to take their operations to a new level, both in terms of simultaneity and in terms of the amount of casualties."

Another London-based security analyst, Sajjan Gohel, of the Asia-Pacific Foundation, says he is not 100 percent certain that ETA even has the capability to carry out such an attack

"It was a coordinated, well-timed, well-planned, multi-purpose, mass-casualty terrorist attack. And that usually involves a transnational terrorist group like an al-Qaida or [an] affiliate," he said. "If this is, in fact, an attack by ETA, then we're looking at a very new departure from its traditional type of attack and a very disturbing level of sophistication in their ability."

Mr. Gohel thinks that ETA might have received assistance from al-Qaida or another such group to carry out the bombings.

U.S. and British intelligence officials note that there was no warning phone call, which ETA usually gives before an operation. They note the high number of civilians killed is unusual for ETA. And they say that such simultaneous bombings are much more the hallmark of al-Qaida than of ETA.

But Spanish and other European security officials say that even though ETA has been hit hard by a government crackdown over the past two years, there is still a radical core that apparently has no qualms about mass killings. One expert says these people could have taken al-Qaida as a model to get media attention and keep their cause alive.

Paul Heywood, an expert on ETA at the University of Nottingham in Britain, says he believes ETA staged Thursday's attack to provoke even stronger action by the government.

"This is a classic tactic by organizations such as ETA, which is to try to set up a cycle of violence where the government response becomes so extreme that it then forces some people into supporting ETA's position precisely on the grounds that there's a perceived imbalance in the response," he said.