You keep remembering and reading basically WRONG stuff.
The total number of FBI agents in the US ranged between 3,000-5,000 (which included non-field agents). The KKK and related groups had many thousands of members at any one time (say 1960-70s when it went into decline, due, in part to the 1965 hearings by the House Committee on UnAmerican Activities which exposed the Klan as the violent, hate group that it is).
The FBI couldn’t assign 3,000 agents to monitor the Klan, etc. That’s not how surveillance/intelligence investigations work. Been there, done that. Most agents worked on organized crime, interstate crimes, esp. kidnappings, bank robberies, hijackings, Soviet and Soviet Bloc and Red Chinese espionage, white collar crime, and aid local police at times.
THere was one radical organization that actually did have several FBI agents or informants in it but that was an anomaly (depended on the possible threat that the group posed and its size). Sometimes the police ran their own intell ops when they perceived a local threat. Been there, done that, only it was another private group that did the infiltrating on its own. The ENTIRE OPERATION, in both cases, consisted of about 12 people, and in my case, consisted of several very violent SDSers.
AND they were not supposed to know what the others were doing. Independently sourced intelligence is much better than single-source intelligence. Been there, done that.
Since you know and remember so much, here’s a story you can add to your repetoire. I was working for a govt agency (ies) and testified about a person who later turned out to be a local police officer in another city. He later testified before Congress about what he had learned, about 6-12 months after my cover was broken so that I could testify before Congress (about radicals/violence/communist domination of key national groups/KGB ties, etc).
This officer provided inside information on a very important Chicago group that I could not have provided since I didn’t operate there. He also provided information which expanded on what I had said about several organizations.
Two independent sources are much better than one.
Remember that, if you will.
I have no idea how many people were in the KKK but I am sure I read that from a normal source. It could have been wrong but I am not wrong about it being reported.
Now the one about the FBI having informants who operated an entire operation from within some radical group by accident, I am certain was widely reported.
I don’t care if you are J. Edgar himself, I just repeated what I read. At one time I would have trusted the FBI to an extreme degree and that trust was probably justified.
Now I think they are a bunch of clowns. Well actually there probably are lots of good ones left however I personally witnessed a bunch of FBI agents behaving like trash during the 60’s.