The FBI Tuesday added its voice to anti-virus firms warning of the Sober.k worm, advising Americans to ignore the mass-mailed worm, which sometimes poses as a tell-us-or-else message from the federal law enforcement agency.

Sober.k, which also arrived as file attachments to messages offering free access to X-rated videos of heiress Paris Hilton and as security alerts from Microsoft, can appear with a variety of FBI-like addresses, including "FBI@FBI.gov" and "police@fbi.gov." The text of such messages reads:

"Dear Sir/Madam, we have logged your IP-address on more than 40 illegal Websites. Important: Please answer our questions! The list of questions are attached. Yours faithfully, M. John Stellford ++-++ Federal Bureau of Investigation -FBI- ++-++ 935 Pennsylvania Avenue, NW, Room 2130 ++-++ Washington, DC 20535 ++-++ (202) 324-3000"

Not so fast, said the FBI. "These e-mails did not come from the FBI," the agency said in a statement. "Recipients of this or similar solicitations should know that the FBI does not engage in the practice of sending unsolicited e-mails to the public in this manner."

In fact, earlier this month, the FBI shut down an e-mail system it used to communicate with the public because of a possible security breach.

The FBI said it was investigating and asked anyone who had received the Sober.k worm packaged in a message supposedly from the Bureau to report it to the Internet Crime Complaint Center.