A mass e-mail purportedly from the FBI is circulating online, carrying with it an attachment that contains a variant of the Sober computer virus.

In an announcement today, the FBI advised computer users that the agency never sends unsolicited e-mails and that they should not open the attachments in the fraudulent messages.

The scam e-mail tells recipients that their Internet use has been monitored by the FBI and that they have accessed illegal Web sites, according to the FBI. The e-mails, which are sent from e-mail addresses including mail@fbi.gov, post@fib.gov and admin@fbi.gov, then direct recipients to open an attachment and answer questions.

The text of the fake e-mails says: "We have logged your IP-address on more than 30 illegal Websites. Important: Please answer our questions! The list of questions are attached." The messages then include a fake name of an FBI official and the real address and phone number of the agency.

No information was available on how many of the e-mails might have been sent or how many computer users might have launched the viruses on their machines.

The FBI said it "takes this matter seriously and is investigating" the scheme.

Recipients of the fake messages are being asked to report them online to the Internet Crime Complaint Center.