They’re already on record as saying that a proper investigation of the laptop email cache should take weeks. They said they’re pushing to try to get something done in days, but to exonerate on such a cursory reading would be tantamount to admitting the fix is in, because there’s no way they could have examined everything so soon. However, it is quite plausible that they could have come across something particularly damning in the early stages an intensive search, even though they’re not done reviewing the entire tranche.
My guess is it’s either something really damning, and they’re acting on it while continuing the search for additional violations, or Comey is bailing in an attempt to extract himself from the Kobiyashi Maru he’s rapidly approaching.
What if they indicted Huma and Carlos Danger but not Hillary?