I have wondered this myself and have looked into it, but have never found any evidence of a specific department within the FBI dedicated to cults.
Reports of FBI investigations into cults neever seem to identify a specific unit - if you do a google search or search Rick Ross you will see what I mean. They certainly do seem to take an interest in cults though.
A few links which may be of interest on this subject, the FBI has investigated numerous cults, many declassified files of high public interest are easily available on-line in the electronic reading room www.foia.fbi.gov
Interacting with "cults" A policing model
FBI Law Enforcemnt Bulletin - Sep 1, 2000
By Adam Szubin, Carl J. Jensen III and Rod Gregg
[
bernie.cncfamily.com]
Project Megiddo : "an FBI strategic assessment of the potential for domestic terrorism in the United States undertaken in anticipation of or response to the arrival of the new millennium."
You can read the report on this project and link to the official file in pdf here : [
www.apologeticsindex.org]
If looking for documents about a specific cult you may want to also check out www.the memoryhole.org