I posted almost exactly this phrase a few days ago.
But, my rationale was that Comey had to be sure his case was airtight, due to Clinton's position.
I hadn't thought about what you wrote:
I think they have to keep investigating until they run down every crime Hillary has committed.
Now, I think you hit the nail right on the head. When the FBI got the entire email archive (or server backup?) from Clinton's lawyer, it was the equivalent of opening Pandora's box.
Every one of those emails containing classified information is evidence of a crime. So, they have to follow it back to the source. Those "interviews" won't be announced in the media.
Then, they have to follow every one of them to any destination OUTSIDE Clinton's server, and determine the final disposition of the information -- compiling the names of every person that received it and compare the classification level against their clearance. Due to the nature of email, this is potentially an exponential level of effort.
And, we haven't yet considered the estimated 30,000 emails that were DELETED. Every one of those deletions is a potential crime even if it's not evidence of wrongdoing: if it is any official business, it was evasion of the FOIA.
If it's evidence of any criminal or civil offense, then the FBI can start with obstruction of justice and build on that. If there is any hint of bribery or washing money through the Clinton foundation, it opens a whole new front for inquiry, and an entirely new set of FBI agents.
Finally, there's another aspect that I don't think anyone has written about: who else used clintonemail.com? Was it just Hillary, Bill, and a few staff members? Or, was it also used by some/all employees of the Clinton Foundation?
I suspect the email/server backup retained by Clinton's lawyer contained more than Hillary's emails. There is likely an entire trove of messages for the FBI to examine, including messages between people other than Hillary.
The FBI may need "probable cause" to look at those, but all it will take is one questionable email between Hillary and another person with an account on that server, and they'll widen the net. And then, they'll start looking at every email that person exchanged, and they'll have a whole net set of interviews to conduct.
The FBI may have found itself "going down the rabbit hole". I don't know how far they will go before they decide whether to recommend charges against Hillary. But, she may only be the tip of the iceberg. ALL of Clinton's cronies may be under FBI scrutiny, and they are probably sweating bullets.
If that's the case, there are a lot of people that have a vested interest in Hillary's election, because it's the only way she can shut down the investigation. Perhaps the FBI wants to charge any co-conspirators before they get protected.
Thanx for the enlightening post