Via governmentattic.org:

• “National Security Counselors law firm has obtained a copy of the CIA Directorate of Intelligence Style Manual, Eighth Edition, 2011.  It is entitled Style Manual & Writers Guide for Intelligence Publications. The CIA Guide is not alone.  Each of the members of the Intelligence Community ­IC ­ have one or more Style Manuals to conform the reports and documents of that agency to a consistent writing style and usage.  This is highly important to achieving clear and unambiguous communications of such matters.
• Here is another example: the NSA SIGINT Style Guide
• The National Security Counselors web site publishes a large number of interesting documents released under FOIA, or under litigation arising from FOIA requests.”