Explaining the FBI's Sunday Announcement About Hillary Clinton-Related Emails

— -- The FBI says it has now reviewed a cache of newly discovered emails tied to Hillary Clinton’s use of a private email server, and the agency says it has still found no evidence that Clinton knowingly sent or received classified information during her time as secretary of state.

So why did the FBI make it all so public, and what was it even about?

What Did the FBI Determine?

In the new trove of emails, the FBI found thousands of potentially relevant messages, including messages sent to and from Clinton herself while she was secretary of state. But the FBI found most of those emails to be personal messages or ones they had already seen.

So “we have not changed our conclusions that we expressed in July with respect to Secretary Clinton,” FBI Director James Comey told lawmakers in a letter Sunday.

In those July remarks, Comey said his agents “went at this very hard to see if we could make a case," but "my judgment is that she did not" break the law.

Nevertheless, speaking before lawmakers in July, Comey described several statements made by Clinton on the campaign trail about her use of the server as "not true." In particular, he said, Clinton was wrong when she said she never sent or received classified information over the server, but the FBI found no evidence that Clinton realized she was sending or receiving that sensitive information.

"I think she was extremely careless. I think she was negligent. That I could establish,” Comey told a House panel. “What we can't establish is that she acted with the necessary criminal intent. 'Should have known,' 'must have known,' 'had to know' does not get you there. You have to prove beyond a reasonable doubt that they knew they were engaged in something that was unlawful."Comey said he knows that "frustrates people, but that's the way the law is, and that's the way the practice is in the Department of Justice."

How This New Cache of Emails Was Found

These emails were found by chance. Earlier this year FBI agents in New York were analyzing a laptop used by former Rep. Anthony Weiner of New York, who was accused of sending sexually explicit messages to an underage girl. During that review, agents came across some emails sent or received by Huma Abedin, Weiner’s now-estranged wife. She is one of Clinton’s most trusted aides, and she was among only a handful of people who had an email address from Clinton’s private server.

What Happened at the FBI Afterward

The discovery of emails potentially relevant to the Clinton-related probe, by investigators not involved in the probe, sparked a complicated process at the FBI, according to sources. For weeks, agents separately working on the Clinton and Weiner investigations and their superiors engaged in a back-and-forth over how to handle the situation.

As first reported by The Washington Post, the process was further delayed because agents were trying to find software that could effectively sort out thousands of Clinton-tied emails from the nearly 650,000 unrelated emails on Weiner’s laptop. At the same time, a key question for agents was whether the FBI needed to obtain a warrant in order to review the emails, since they were discovered in the course of a separate and unrelated investigation.

The investigators working on the Clinton email probe put together a summary of the situation, and 10 days ago they presented it to Comey, who agreed with their assessment that a warrant was needed. Comey sent lawmakers a letter the next day, informing them of further “investigative steps” in the matter. In his latest letter Sunday=, Comey said a team of FBI agents “has been working around the clock to process and review [the] “large volume of emails.”

Why Did Comey Do This Publicly?

In his July testimony to lawmakers, Comey said, under oath, that the Clinton email-related probe had been “closed,” and that no further investigative action was being taken. The discovery of new emails on Weiner’s laptop, however, changed all that.

“[W]e don’t ordinarily tell Congress about ongoing investigations, but here I feel an obligation to do so given that I testified repeatedly in recent months that our investigation was completed,” Comey said in a message to FBI employees nearly two weeks ago. “I also think it would be misleading to the American people were we not to supplement the record.”

ABC News’ Pierre Thomas, Geneva Sands, Jack Date, Lucien Bruggeman and Ali Rogin contributed to this report.