Terror-alert system: how it's working

As the national color-coded system took a departure with these newest warnings - leaving not just geographical areas but individual employees of specific organizations as targets - analysts are weighing its effectiveness against the psychological impact on both the public and the potential perpetrators.

For some, the warnings, even though specific, still cause unnecessary panic - even beyond the target areas. Security measures have been ramped up in cities across the nation. Robert Butterworth, a trauma psychologist in Los Angeles, says the alert system creates "anticipatory anxiety," in which the public is scared long before anything happens.

"If you hear there is going to be a bank robbery, do you announce it on the radio?" says Dr. Butterworth. He says officials should put precautions in place, "but don't broadcast it to the world" - especially since this threat is not believed to entail large-scale biological or nuclear weapons.

But others say that the government, if operating with credible information, would have had no choice but to release the information to the wider public. "It would be very difficult to say to the people, 'We have a credible threat to your building, but we don't want you to talk about it,' " Mr. Pfaltzgraff says. With the thousands of people who work in those buildings - witnesses to the ramped-up security - secrecy would create more alarm.

To a certain extent, the information released could tell terrorists what US officials know: what top level members of Al Qaeda have been captured or what channels of communication they have accessed. It can also jeopardize sources, says Pfaltzgraff.

There's also the possibility that terrorists could purposefully mislead US officials. They could be looking at secondary targets. "Everyone is looking at truck bombs, car bombs, and suicide bombers," says Mr. Larsen. "How about if they planned a different kind of attack?"

The US pronouncements of specific information may be a brief setback psychologically to the terrorist network, but analysts say it doesn't necessarily have a demoralizing effect. In fact, such setbacks could serve as challenges. "There's going to be a core group of people who want to do it in any event, and might even view it is a dare to see if they can actually do it," says Kayyem.

But so long as a threat exists, many say the specific information is a positive departure from the color-coded system, in which, in theory, a terrorist attack was just as likely to occur on the East Coast as the Midwest plains. "It had been so broad, so nebulous," says Kayyem. This time, officials are telling the public "who should be nervous and who should not be. And that is important."

Anthony Cordesman, an expert at the Center for Strategic and International Studies, says the information points to a more integrated intelligence system.

• Ron Scherer in New York contributed to this report.

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