Classification is the predominant means of protecting national security information. But even when information is unclassified, there are a number of statutes that can be used to restrict its public availability on security-related grounds.
Such statutory controls on unclassified security-related information are usefully cataloged in a new report from the Congressional Research Service.
See “Protection of Security-Related Information” (pdf), September 27, 2006.
For no extra charge, here are a couple of other recent CRS reports (pdf) obtained by Secrecy News.
“The Use of Federal Troops for Disaster Assistance: Legal Issues,” updated August 14, 2006.