In a 24-hour news cycle dominated by fears of terrorism, the latest e-mail from the Abu Hafs al Masri Brigade to a London-based Arabic newspaper sowed anxiety and drew instant headlines all over the world.

But specialists say there is no evidence the organization exists. E-mail messages purporting to be written by the group previously claimed responsibility for everything from the North American blackout to a suicide attack that killed 20 Italian policemen in Iraq. But none of those claims has proved true, intelligence specialists say.

The latest message warned that an attack against the United States is "90 percent ready."

Employees at the Al-Quds Al-Arabi newspaper in London, which has received several letters in recent months purporting to be from the group, say they are not sure what the group is.

Deciphering the group — which first surfaced in July 2003 — illustrates a larger challenge for counterterrorism officials: assessing the murky world of purported Al Qaeda splinter groups that have been responsible for a dramatic increase in highly sophisticated terrorist bombings around the world.

Spanish officials said yesterday they had no concrete evidence of Al Qaeda involvement in the Thursday attacks, and continued to identify ETA — a Basque separatist group — as one suspect.

But the worldwide attention generated by the Abu Hafs al Masri e-mail, received hours after the attacks occurred, demonstrated how easily threats purported to be from Al Qaeda can be spread. Terrorism analysts say such claims form part of the tactics of psychological warfare and propaganda, designed to capitalize on actual violence and deepen public fear of more attacks.

"It goes to the more virtual nature of Al Qaeda," said Peter Bergen, a terrorism specialist at the New America Foundation in Washington. "Some are real; some are waging psychological warfare."

A US official who has access to the latest intelligence information said the brigade’s name is the nom de guerre of a senior bin Laden lieutenant who was killed by American forces in an airstrike in Afghanistan in November 2001.

"The name they picked out is what is sometimes confusing. Their ties to Al Qaeda are not as clear-cut," said the official, who asked that his name not be used. He added that US intelligence officials do not know whether the group exists.

Another US intelligence official said, "This group, organization, or whatever it is, is a phantom."

Ben Venzke, CEO of IntelCenter, a private company that specializes in analyzing terrorist messages for government agencies, said that while he believes Al Qaeda could have been responsible for the Madrid bombings, messages from the purported brigade are not credible.

Venzke said it became clear last year that the many letters and faxes did not have the same style as messages from Al Qaeda. Bin Laden’s organization and offshoots have become more media-savvy in recent years and are increasingly providing video or audiotapes that can be verified for authenticity, Venzke said.

For example, e-mail messages to a journalist for Al-Majallah magazine who is based in Dubai, United Arab Emirates, sent days before the May 13 bombings in Saudi Arabia are thought to have come from a bona fide messenger of Al Qaeda. The e-mails warned of impending guerrilla warfare on the Arabian Peninsula. The attackers even made videotapes of the preparations for the bombing to be sent to the public.

But unlike those messages, none of the brigade’s e-mails has ever proved authentic, Venzke said.

By last August, he said, he already had begun to disregard the group when it sent the clearest indication that it is a sham: a message to Al-Quds claiming responsibility for the blackout that struck New York City, Ohio, southern Ontario, and eastern Michigan last year.

That blackout was determined to have been caused by a power grid failure.

Larry Mefford, executive assistant director for counterterrorism at the FBI, told Congress that the group’s terrorism claims were "wishful thinking."

"We have no information confirming the actual existence of this group," Mefford said, adding that the group also has claimed responsibility on Internet sites for the Aug. 5, 2003, bombing of the Marriott hotel in Jakarta, Indonesia.

Venzke said: "They started claiming responsibility for just about everything in the world. We’ve never been able to determine if it is just one person sitting at a computer having fun or if it really is a group."

The editor of Al-Quds, Abdel Bari Atwan, could not be reached yesterday, but a woman who identified herself as a receptionist said that e-mail messages from the brigade came in Arabic and that she has no other information about the group. When asked whether the letter seemed credible, she said: "That’s the problem. Probably not.

"There is no proof of anything, if such a thing exists or is being made up by America or someone else."