Nearly two months after the devastating attacks on New York and Washington, Western intelligence and law enforcement officials say they are convinced Islamic terrorist groups connected with or sympathetic to Osama bin Laden's al-Qaida network could strike again. The officials say they do not know where the attacks will take place or whether they will involve explosives or chemical and biological weapons, but they are certain of one thing: more attacks are coming.

According to the latest threat assessment circulating among European law enforcement agencies, U.S.-led military operations in Afghanistan and the clampdown in Europe on suspected supporters of Mr. bin Laden have not diminished the danger of further terrorist action.

Security officials working in five European countries say their priority now is to search for new leads that might help unravel the full extent of the al-Qaida network in Europe and terrorist units that may or may not be linked directly to it.

Security experts like Matthew Dunn, an analyst at Control Risks Group in London, says one of the problems investigators face is that there does not seem to be a formal, hierarchical structure to al-Qaida. "I'd say these groups are fairly informal and have some links with al-Qaida, or maybe receive funding and training in Afghanistan from al-Qaida, but have some degree of autonomy about how they go about carrying out attacks, in fact, which attacks they carry out," he said.

Analysts note that, in the past, there were many months between attacks launched by al-Qaida; two years, for example, between the 1998 bombings of two U.S. embassies in Africa and the attack on the U.S. destroyer Cole last year in Yemen. But some intelligence officials say the expectation of Osama bin Laden's followers and supporters is that he will respond to the U.S.-led attacks against his allies, Afghanistan's ruling Taleban militia.

Steven Simon, an expert on al-Qaida who served on the U.S. National Security Council during the Clinton administration, is now at London's International Institute for Strategic Studies. He says it is impossible to know what Mr. bin Laden will do next. "Bin Laden has typically taken time between attacks to make sure that everything is okay and that things work well, recognizing, of course, that a successful attack relies as much upon luck as anything else. And, of course, on September 11, the group got exceedingly lucky," he said.

Mr. Simon says Mr. bin Laden recently sent a videotape to the Qatar-based Arabic-language satellite television network, Al Jazeera, in which he promised to hit the United States and its allies again. "What he typically does is announce an attack before he carries it out, which is a reversal, of course, of the standard procedure, if you want to put it that way, of claiming responsibility for something after the fact. I'd say he's probably capable, and he's highly motivated and if he can do it, he will. But he's not going to do it, unless he thinks it will really work and have a devastating effect," he said

Intelligence officials are taking very seriously a warning issued last week by the International Atomic Energy Agency [in Vienna, Austria] that terrorists could obtain nuclear materials or crash a hijacked airliner into a nuclear power plant. Though most officials do not believe al-Qaida could obtain the means to build an atomic bomb, they have few doubts that the group is willing to cause as much devastation and panic as it can through the use of a so-called "dirty bomb," in which conventional explosives are wrapped around radioactive materials.

U.S. and European officials have had some success against al-Qaida in recent years. They have prevented several major attacks, such as the simultaneous explosion of 11 airliners in Asia, the millennium plot to blow up the airport in Los Angeles and strikes against the French city of Strasbourg last December and the U.S. Embassy in Paris this year.

But Jonathan Stevenson, a counter-terrorism expert at the International Institute for Strategic Studies, has said those successes are forgotten in the wake of September 11, and that the group remains as deadly as ever. "I think you have to say that, as terrorists go, that al-Qaida is a successful and unprecedentedly dangerous terrorist organization, both in terms of the scope of its operation and its lethal intent," he said.

And that, say U.S. and European officials, is why they cannot afford to let their guard down.