[plain talk]

So the FBI can read the email headers but not the email text content? Is that how it works? I assume they can read the headers otherwise how would they know the emails have anything to do with Hillary ... and thus a need to reopen her investigation?

[Mariner]

They have read ALL the emails and know what they contain. They were incidental evidence in another properly executed warrant.

They seek ANOTHER warrant specific to those emails to ensure an unquestioned path to prosecution.