The FBI was chartered to investigate crimes. Constitutionally, all investigations of people must begin with knowledge of crimes. Constitutionally, the FBI may not investigate a person without “probable cause” that the person committed a crime which the FBI has knowledge. And all evidence which the FBI collects which is not gathered this way becomes “fruit of the forbidden tree”” and becomes inadmissible in court.
The problem with phrases like “probable cause” is they can be unconstitutionally twisted and expanded to mean ANYTHING a corrupt FBI official wants it to mean, up to and including parents who are now “probably” domestic terrorists because they are fighting the school boards over the way they are educating their children ( especially if the parents are white ).