Suggestion for starting: You will likely want to start with the State Department's Bureau for Counterterrorism and Countering Violent Extremism.
You can search Google for government publications, and it can be extremely helpful for identifying relevant agencies/committees that have a vested interest and/or produce counterterrorism policy.
To search .gov sources on Google, use the following search:
site:.gov
For example: site:.gov counterterrorism policy