Dept. of Defense Directive:
http://www.fas.org/irp/doddir/dod/d5205_02.pdf
4.3.1. Adversary intelligence collection threats include the exploitation of publicly available information often obtained through open networks and information on websites. These and other detectable unclassified activities are used to derive indicators of U.S. intentions, capabilities, operations, and activities.
A necessary condition for maintaining essential secrecy is protection of classified, as well as unclassified critical information. This protection ensures that, beyond the application of traditional security measures, the Department of Defense maintains a heightened awareness of potential threats.
Before the terrorists attack, their Internet chatter almost ceases. They have learned how to avoid the government’s surveillance.