The fictitious e-mail indicates the FBI has collected information proving that the user has visited illegal Web sites. The e-mail instructs users to open the attached document to answer a list of questions. But instead of a seeing the list of questions, users infect themselves with the latest version of the Sober worm.

The e-mail appears to come from addresses such as mail@fbi.gov, post@fbi.gov, and admin@fbi.gov. The FBI cautioned that it never sends unsolicited e-mail to the public and is urging computer users who receive the e-mail to report it to the Internet Crime Complaint Center at www.ic3.gov.

Guilt and Gullibility

According to Graham Cluley, an analyst at Sophos, the Sober worm accounts for 65 percent of all viruses reported to the company's global network of monitoring stations. To put this in perspective, Cluley said, it means that one in every 74 pieces of e-mail sent over the Internet -- including legitimate messages -- is currently infected by the Sober worm.

"That, in itself, is a sobering statistic," Cluley said. "This is an example of mass-scale malware distribution."

Spreading the virus using a phony e-mail from the FBI containing allegations about illegal Web sites is a disguise that the bulk of recipients will never investigate themselves, said Cluley.

"The point of using the FBI disguise is that the vast majority of people will never check the FBI Web site to read the warning before opening the e-mail attachment," Cluley said.

Guilt Trip

Rob Ayoub, an analyst at Frost & Sullivan, said computer users must be smart and reasonable about these innovative threats. If they receive an e-mail that says the government has been tracking them, they need to weigh the likelihood of that possibility.

"People need to be reasonable and smart about what they are doing," Ayoub said. "Just because something says the FBI, people should not react to that. People need to ask themselves if what they have done online is worth an FBI investigation." Ayoub went on to say that this scam underscores the need for ongoing education for computer users.

"If a guy in blue jeans and a t-shirt came to your door and said he was sent from UPS to pick up a package, would you believe him? Probably not," Ayoub said. "People need to use that same skepticism on the Internet."

Security firms fear that in an effort to avoid being falsely accused -- or simply to help law-enforcement officials -- computer users will unthinkingly open the virus-ridden attachment.

"This variant of the Sober worm might catch the unwary as they open their e-mail inbox this morning," said Cluley. "All users should be reminded to follow safe computing guidelines, and PCs should be kept automatically updated with the latest antivirus protection."