Posing as the US Department of Justice, a new variant of the Citadel Malware called Reveton is responsible for a ransomware campaign that attempts to extort $100 from its victims. The infections are occurring after users are lured to a drive-by download site where a dropper installs the Citadel malware. The Citadel strain locks-down its host computer, displaying a fake message warning users that their computer has been identified by the Computer Crime and Intellectual Property Section of the US DoJ for having visited websites containing child pornography. In order to unlock their machines, users are prompted to pay a $100 fine to the DoJ.
Source: https://threatpost.com/malware-poses-us-department-justice-violation-notice-050112/76506/