Information on Grand Jurys which are empaneled by US Attorneys in criminal matters:
While grand juries are sometimes described as performing accusatory and investigatory functions, the grand jury's principal function is to determine whether or not there is probable cause to believe that one or more persons committed a certain Federal offense within the venue of the district court. Thus, it has been said that a grand jury has but two functions—to indict or, in the alternative, to return a "no-bill." See Wright, Federal Practice and Procedure, Criminal Section 110.
All Huber has to do is obtain an indictment from the Grand Jurys he has empaneled.
The bar for indictment in notoriously low. Huber probably did not need to interview anyone. CF crimes are so obvious that they did not require interviews beyond those Horowitz carried out.
Indictments remain sealed in order to preserve other investigations and to also allow for arrests at the time it most makes sense for law enforcement.
STEALTH HUBER
https://www.justice.gov/jm/jm-9-11000-grand-jury#9-11.101