A hosted e-mail system that the FBI uses to communicate with the public has been taken out of action after it was "compromised".
The FBI said the fbi.gov e-mail system would be out of use until it had completed an investigation into the issue.
"We are aware of a compromise to a commercial e-mail system which hosts fbi.gov accounts, provided by a commercial supplier," it said.
"These accounts are used for non-sensitive, non-secure communications and do not impact secure internal and external FBI e-mail accounts. The FBI has suspended use of these fbi.gov accounts pending completion of a review of the matter."
Last December the FBI joined forces with AOL, Microsoft, and other IT suppliers and government agencies to form an e-mail security group, which would tackle growing e-mail threats such as phishing.