MADRID, Spain, 14 March 2004 — The size and savagery of Thursday’s synchronized train station bombings suggests a significant shift in tactics by Basque separatists — or the work of an entirely new player in Spain.

Spanish authorities are wrestling with competing theories over who was responsible for the attacks that killed nearly 200 people and wounded approximately 1,400 others. Swift to blame ETA militants fighting to secede from Spain, top officials by the end of the day were forced to acknowledge that they were pursuing a more complex investigation.

Forensic evidence, including the type of explosives used, and the arrests in recent weeks of purported Basque terrorists armed with powerful bombs point to ETA as the prime suspects in Thursday’s massacres, investigators said in interviews, echoing Interior Minister Angel Acebes.

ETA purportedly planned a Christmas Eve bombing at another Madrid train station, which was foiled by police. Recent intelligence reveals that a young, ruthless generation of Basque militants has been pushing since the summer to pull off a spectacular blow, according to a senior Spanish law enforcement official.

But the well-coordinated multiple strikes aimed at mass civilian casualties are elements out of character with ETA’s three-decade-old armed campaign. ETA traditionally has targeted the symbols of Madrid’s power, such as police and politicians, or its economic base, with hits on tourist resorts.

The methods Thursday recalled the brutal choreography refined by Al-Qaeda in its Sept. 11, 2001, attacks on New York and Washington, D.C., and more recently around the world. And the discovery of Islamic audiotapes along with detonators in a stolen van thought to have been used by the attackers forced investigators to consider a grim alternative scenario: Islamic terrorists might have chosen Spain for the attack, their first successful strike in Western Europe.

“We are pursuing both the possibilities of Basque terrorism and Islamic terrorism,’’ a Spanish police commander said. “ ETA remains the stronger possibility. But these attacks were rather strange. There are elements that resemble Al-Qaeda, elements resembling ETA, and some things that are neither one nor the other.’’ The method of attack fed a third theory: That the coordinated strikes resulted from an alliance of Basque and Islamic networks.

In past decades, ETA’s worldwide contacts have brought it into contact with Arab militants in locales including Libya and Lebanon. Last year, attacks by suicide bombers in Casablanca, Morocco, and Istanbul, Turkey, showed the ability of Al-Qaeda operatives to join forces with local Islamic groups. Most investigators regarded a partnership between the secular nationalists of ETA and today’s violent Islamic fundamentalists as unlikely.

Nonetheless, the calculated plan to inflict massive casualties suggests that Al-Qaeda has become an influence for post-Sept. 11 terrorism and might have been copied by ETA, authorities said.

“Maybe what we are seeing is an ‘Islamicization’ of the style of ETA terrorism,’’ said an investigator of Spain’s paramilitary Guardia Civil. “That is, they see that spectacular, coordinated attacks like those carried out by Al-Qaeda are the ones that cause the most impact. So instead of killing a city councilman somewhere, they do this.’’ The investigator said Thursday’s attacks had no such value. “The targets were not symbolic: Just working people taking the train at 7:30 a.m, ‘’ he said.

ETA has its own brand of ruthlessness. One trademark, which police detected again Thursday, involves setting time bombs at attack scenes that are designed to kill police and rescue personnel arriving shortly after.

Nonetheless, ETA’s attacks, which have killed about 800 people in a period of more than 30 years, have exhibited a certain method. Normally — although not always — an ETA member telephones a warning shortly before an explosion. The most people ETA ever killed with a bombing was 22 at a Barcelona supermarket in 1987. The group later apologized for killing so many civilians. During recent years, similar divisions surfaced as aggressive police work chopped away at ETA’s old guard leadership, among them veterans old enough to remember the years of the Gen. Francisco Franco dictatorship when ETA’s fight against the regime elicited some sympathy in Spain, according to the official.

The old guard has been pressured by young toughs intent on reinvigorating a weakened and widely despised group, the official said.

“At the end of the summer, there was intelligence that the moderate sector had rejected pressure from the more radical ones pushing for a big massacre,’’ the official said. “This is a generation of 20 to 30 years old. They were born and raised in democracy, yet they are the most radical of all. And basically (they are) street thugs, without ideology, really brain-dead. The older ones have a certain philosophy when they talk to you, a political discourse. Not these guys.’’

More recently, police arrested ETA suspects near the city of Cuenca with more than 1,000 pounds of explosives destined for an industrial neighborhood near the railway station where the targeted trains apparently were boarded by bombers Thursday, according to investigators. About 10 to 12 kilos of explosives were used in each of the 10 strategically placed backpack bombs Thursday, investigators said. The kind of timers and the explosives used closely resembled the bombs in past strikes by ETA, investigators said. The timing of the attacks fit ETA’s history, coming as they did just three days before national elections. ETA often waits to claim responsibility after an attack, evaluating the political fallout. Because Thursday’s attacks caused such carnage, they seemed politically self-destructive, which could explain why members of ETA’s outlawed political wing declared that the attacks were the work of Arab terrorists.

Although investigators were dubious about the legitimacy of that denial, they acknowledged that the style of the attacks raised the alternative that Al-Qaeda or its affiliates had been involved. Al-Qaeda has been active in Spain for years: A network disrupted in late 2001 was accused of aiding the Sept. 11 hijackers. Extensive wiretaps of the network never indicated any plans for an attack in Spain, which Al-Qaeda values as a transit point to North Africa and Europe.

But Spain has been an aggressive partner in what Washington calls its war on terrorism. Prime Minister Jose Maria Aznar was a prominent supporter of the war on Iraq, a conflict whose first anniversary approaches. In an audiotape last fall, Al-Qaeda’s Osama Bin Laden singled out Spain and Italy, another staunch supporter of US policies, as potential targets for punishment. Several weeks later, 19 Italian officers were killed by a suicide bomber in Iraq.

A group calling itself the Brigade of Abu Hafs Al-Masri e-mailed a claim of responsibility for Thursday’s bombings to an Arabic newspaper in London, saying it was “settling old accounts with Spain, the crusader, and America’s ally in its war against Islam.’’ Authorities were unsure whether to take the claim seriously.

In Washington, the State Department was emphatic Thursday in its rejection of suggestions that the bombing was the work of Al-Qaeda. The department spokesman said Spain believed ETA was responsible and that the US government had no evidence to indicate otherwise. The only hard clue suggestive of Al-Qaeda-style groups is the material found in the stolen van that also contained seven detonators. The material consisted of audio cassettes with recorded verses of the Qur’an, easy to find in an Islamic butcher shop or Islamic bookstore, the police commander said.

“It’s possible, of course, that ETA could be responsible and the cassettes were left there to throw us off the track,’’ the commander said.

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