I like to learn and I wanted to learn more about the movie Imperium. It turns out that the movie is somewhat based on some of the life of FBI agent Mike German. He did his undercover work in the 1990s and his uncover cases were in California and then in the Pacific NW.
In the California case, the criminal activity was being planned by a group that was nearly unknown or completely unknown to law enforcement prior to his getting involved, it appears.
In both cases, he was introduced to the white supremacist groups because a person among them saw things they thought were evil or likely to be evil and needed to be stopped.