The police and FBI, etc., are limited in what they may lawfully record even when carrying out official investigations of criminal activity. Audio surveillance requires a warrant, and it is potential police and/or prosecutorial misconduct (which can get a criminal case thrown out of court on a defense motion) if surveillance occurs without one. If you're not a prosecutor, police officer or detective and you're not carrying out an official investigation, you can't assume the authority to conduct surveillance.
I'm not a lawyer and I'm not asking you to take my word for any of this. I'm just trying to say why you may want to consult an attorney before doing something that could get you into potentially heavy legal trouble. In some situations, recording a conversation to which you are a party isn't a criminal violation and wouldn't expose you to any civil action--but divulging the contents of the recording to any third party (or recording a conversation to which you're not a party) would step over that line in a major way. In some situations the means of communication (e.g. use of Federally regulated telephone systems) becomes a potential legal issue.
It's not like on TV or in the movies; there's no special legal protection for citizens who take vigilante action.
--best regards