FBI and Most other Federal Agencies have a Standardized frequency system nationwide, whats used in Dallas is almost the same that is used in Houston or L.A. or Miami or Timbucktoo.
FBI Use Digitally Encrypted Radio's which means you are likely not going to ever, ever monitor them. beit on a Uniden, R/S, Alinco or AOR Scanner.
most people have found it to be a waste of time to even bother programming there assigned frequencies in a radio. and if they think you might be scanning them, they have a nasty lil communications device called a NEXTEL that you will surely not be scanning.
There is a reason why they do this. They are the leading enforcement agency protecting you and I from anti-american radicals who enjoy killing american citizens. those radicals seem to be well funded and may have access to far better or much more sophisticated monitoring devices then you or I might get from Radio Shack, so, while they track those yo-yo's down they do not want to have the state or country listening to their Field Operations.
I hope this helps. but if you are a Young new scannist, just type into Google "Federal Frequencies" and you will un-doubtably get far more then your printer could ever print in a lifetime.
Good Luck!