Investigation is either made completely public after the fact or partially marked secret for certain years. Now if someone intentionally hide the investigation details, well, that's a whole different story and I wouldn't be surprised at all. Writer chooses what to write in their investigation log reports.
Anyway, we really don't know what method they used to gather the information and what kind of information they pulled off for sure.
Investigation is either made completely public after the fact or partially marked secret for certain years. Now if someone intentionally hide the investigation details, well, that's a whole different story and I wouldn't be surprised at all. Writer chooses what to write in their investigation log reports.
Anyway, we really don't know what method they used to gather the information and what kind of information they pulled off for sure.