Even more interesting about those letters, was the specific request to keep this query secret, and not reveal it to the subjects being investigated, "as the FBI's investigation may be jeopardized by this type of disclosure."
The letters were all signed by Charles Kable, the Section Chief of the FBI's Counterintelligence Division.
Also included, FBI communications with the State Department, asking that agency to preserve emails that were sent to clintonemail.com.
The names of 19 individuals were listed in the FBI letter to the State Department Inspector General - all those names were redacted in this FBI release.
The same letter was also sent by the FBI to Secretary of State John Kerry.
The newly released records indicate the FBI went so far as to serve the letter addressed to the State Department Inspector General and the Secretary of State, though it seemed more of a formality for Kerry.
"The preservation letter was served at U.S. Department of State's Visitor Center," read one of the released FBI documents.
Also included in the documents are email exchanges between the FBI and State Department over how to treat some of Clinton's emails that were being released before the election.
"Attached is an email forwarded to us by State Dept. for coordination," reads one email that had the subject line of "State Department Emails - FOIA Coordination."
"The email concerns Benghazi. It is from former Sec. Clinton's emails," the note adds. The name of the sender and the recipient of that email were redacted.
That was part of a series of email exchanges between the FBI and State Department on how to deal with the release of certain Clinton emails under the Freedom of Information Act.
"I've called the State's Legal Advisor's Office a number of times and haven't connected," read one of the many emails released.
"Just received a call from State," read another. "They want to argue about the b1 portion," referring to one of the classifications.
The emails discussing what to do about Clinton's own emails were also subject to similar classification issues, as those notices dot the margins of the FBI's release.
The FBI release also includes an email from the lawyer for Brian Pagliano, the former aide who helped Hillary Clinton set up and maintain her private email server - it notified officials in August of 2015 that he would not be cooperating with the investigation.
The FBI documents also detail the search for information from Platte River Networks, the high tech company that dealt with the Clinton server.
On August 12, 2015, the FBI took possession of a Dell Poweredge Server from that company - the box "Collected/Seized" was checked.
The FBI documents show the breadth of the investigation into the Clinton email matter growing dramatically - whereas the FBI at first was asking for email records from 19 different names, by August 18, 2015, that had grown to 422 in a "Request for Preservation of Records."
The recipient of that letter was unclear; the address and name was redacted by the FBI.
Another letter from August 18, 2015 asked for records to be preserved for over 900 people - again, the recipient of that letter is unknown, redacted by the FBI.
As with other requests, the FBI asks that the recipient not reveal the FBI investigation.
There are also other intriguing documents, like this one - which hint at some kind of tip related to the Clinton investigation.
In this release, which was made on Sunday night without any publicity, the FBI did not release any emails to or from Hillary Clinton.
You can look at the new items yourself, in the FBI Vault.