What is going on?

Jack Goldsmith and Benjamin Wittes write at the Lawfare blog,

Does the letter mean the FBI has found something explosive and seriously incriminating about Clinton?

No. A lot of commentators, particularly on the conservative side, are assuming that these emails must contain something really big for Comey to have acted as he did. We think that misreads the situation.

The more likely scenario, in our view, is that Comey’s actions simply reflect the volume of new material and the consequent uncertainty as to what it might mean. The New York Times reports, citing a senior law enforcement official, that we are dealing with tens of thousands of emails. That means the Bureau will have to figure out how many are duplicative of emails they’ve already looked at, how many are potentially relevant to an investigation limited to the mishandling of classified information, and whether any at all contain classified information. And as noted above, email investigations are extremely complex.

Read more here.