Nunes confirms Andrew Weissmann (youtu.be/uVr9Db_FxHk), and since Aaron Zelby is the only other 'FBI' person with the SC that I can confirm (other than Mueller), odds are high that he's the RETIRED agent. NY Times confirms Zelby is considered Mueller's closest associate and was his FBI Chief of Staff.
https://www.nytimes.com/interactive/2018/11/30/us/mueller-investigation-team-prosecutors.html