LONDON, 15 March 2004 — If it was Islamist militants who struck out of the blue in Madrid last week, then it will dash hopes that Western security forces had blunted the threat from Al-Qaeda since Sept. 11 and all Europe is at risk. Despite billions of dollars spent tracking Islamic radicals over the past 30 months, despite destroying their Afghan bases and putting thousands of agents on the streets, Western spies heard not a trace of “chatter” from Muslim militants before the bombers killed 200 rail commuters, security analysts said.

While responsibility remained unclear yesterday for the worst ever guerrilla attack on a European city, a claim from a purported Al-Qaeda ally and the arrests of three Arabs caused Spain to play down early accusations against Basque separatists.

The broader implication, experts say, is that security is still worse — perhaps far worse — than policymakers feared, even in countries like Spain with experience of dealing robustly with political violence and which were well aware their support for US policy in Afghanistan and Iraq had made them targets.

There is little the authorities can do now to make the public safer, analysts say, apart from continuing with existing tactics — and hoping they have more success next time.

“I wouldn’t say that they were complacent about it. Everybody has been expecting this. But they were just sort of hoping that it wouldn’t happen. This is the worst nightmare,” said Tim Ripley, of the Center of Defense and International Security Studies at Britain’s University of Lancaster.

“Not only are security chiefs very worried about not knowing what to do next, but the general public know that.”

For exactly two and a half years, until March 11, authorities in the West thought they had reason to be proud.

Since Sept. 11, 2001, Islamic radicals had struck in Indonesia, Turkey, Kenya, Saudi Arabia and Morocco, often at targets linked to Western interests, from American workers to Australian or Israeli tourists and a British consulate. Russia and postwar Iraq have also been killing fields for the bombers.

But in the West itself, any major strikes were thwarted. Cells were infiltrated, phones tapped and leaders jailed.

No one would admit to being complacent, but until this week there was a strong feeling that an unprecedented global security effort was proving effective.

Although security services all say they expected an attack would come eventually, many thought that when it did they would at least have a clue in advance.

The absence of any “chatter” — from e-mail monitoring, telephone tapping and other spying — that pointed to Islamic radicals was one reason the authorities so quickly pointed the finger at Basque separatists, an older, better-understood foe.

That conclusion now seems, at the very least, hasty. Worse, there are no obvious answers as to what to do next. In the wake of Sept. 11, the agenda was clear: Improve airline security, commit more resources to monitoring known extremists and destroy Al-Qaeda’s bases in Afghanistan.

But commuter railways, like those attacked in Spain, are all but impossible to protect with the sort of security used at airports. And there is no overseas regime, like the Taleban in Afghanistan, that can now be punished for supporting Al— Qaeda.

The absence of advance clues despite all efforts to snoop is a reminder of how elusive Al-Qaeda— or like-minded imitators — can be, said Marco Vicenzino of the Washington office of the International Institute for Strategic Studies:

“They don’t need to communicate with headquarters. They’ve been probably living there for a long time. They mix into the society they live in. They can scout targets, plan in secret. And then: Bam! They strike. That’s Al-Qaeda and its affiliates.”

If Islamic extremists were indeed behind the strikes, they still may not have involved any of the major Al-Qaeda figures known to be operating in Europe. An Al-Qaeda “spokesman” who claimed responsibility on a tape appears to be someone previously unknown to the authorities. That, said Paul Rogers, terrorism expert at Bradford University, is the nature of the movement.

“It’s actually more difficult than a couple of years ago. Because of the destruction of the Taleban regime in Afghanistan, Al-Qaeda and its affiliates are much more dispersed,” he told Britain’s Sky television. “They are spread much more widely across the world in a loose set of affiliations.”

The fear now is of a rapid spread of attacks throughout Europe. Vicenzino said a wave of attacks across the continent of Europe was now a “strong likelihood”.

Spain, because of its proximity to North Africa, appears to present particular security challenges.

Further possible Islamist targets include other US Iraq war allies like Britain and Italy, but also war critics like France, home to Europe’s biggest Muslim community and singled out last month for criticism by a top Al-Qaeda leader. Germany, base for several Sept. 11 plotters, may also be at risk.

But Western Europe’s open borders make it difficult, if not impossible, to isolate the problem in a single country.

“If it’s an Al-Qaeda connection, we must assume Spain is the entry door for the whole of Europe,” said German terror expert Berndt Georg Thamm. “What happened yesterday in Madrid can happen tomorrow in Paris and the day after in London.”