Federal Bureau of Investigation Network

From GovITwiki

This network, commonly referred to as the FBINET, is a global-wide area network used for communicating secret information, including investigative case files and intelligence pertaining to national security; it also runs administrative applications.

(Description comes from from General Accounting Office document GAO-07-455

The GovITwiki Intelligence Agencies page lists the U.S. government's major secret and sensative but Unclassified networks.