Suppose that the FBI obtains a Title I FISA warrant on a John Doe. That is they argue successfully before a FISA judge that there is probable cause to believe that John Doe is a foreign agent. This grants the FBI broad powers to surveil John Doe.
To what extent can this authority extend to surveil US citizens who John Doe is believed to have contacted? Presumably if John Doe makes a phone call with another US citizen, that phone call will be recorded under the authority of the relevant FISA statute. But what if John Doe speaks with Jane Doe in a manner that suggests she may offer important insight on John Doe's activities. Can the FBI under the original Title I Fisa warrant surveil Jane Doe to some degree?
This is not simply a legal question, but a question of practice. My understanding is that much of the legal precedent and standard interpretations of FISA law is itself classified, and that therefore this may be a difficult question to answer.
The motivation for the question is probably clear. There was a Title I FISA warrant approved to surveil Carter Page, who had some degree of contact with the Trump campaign. If the "dossier" providing evidence of Page's criminality were taken at face value, this would seem to give the FBI broad interest in the Trump campaign as part of the Carter Page investigation. Is there any reason to believe the FBI would not believe they had the authority to surveil the Trump campaign/transition/administration as a result?
I'm not trying to encourage fanciful speculation, but attempting to understand what is plausible.