The Feebs have no choice. Weiner's investigation began with a special task force of the NYPD; it was not Federal. People outside of the FBI have now seen things that Comey and his co-conspirators tried desperately to hide with their immunity deals and spoliation of evidence.
James "Jimmy the Weasel" Comey is an 0bama lapdog. The announcement of the reopening of the investigation is an attempt to get the US attorneys and FBI to take control of the evidence already in NYPD's hands. They're trying to perpetuate the cover-up by making this "announcement." It's damage control.
Perhaps the FBI can, by investigating, demand and obtain the evidence. Then lose it.
zounds, by jove!
:-)