It's the investigation that doesn't seem to die.

Just a few weeks ago, FBI Director James Comey replied to Congress members asking about reopening the investigation of Hillary Clinton's use of a private email server that "I haven't seen anything that would come near to that kind of situation."

The assertion seemed to bring to conclusion a case that led Comey to announce in July that Clinton had been careless but did not break any laws.

Then came Friday, and Comey's surprise announcement that the FBI would, indeed, dip back in — to review a batch of emails "that appear to be pertinent" to the Clinton case, which were unearthed in an unrelated case. He said he wanted investigators to look at them "to determine whether they contain classified information."

Related: Emails Related to Clinton Case Found in Anthony Weiner Investigation

And so now the probe that Clinton so badly wants to put behind her — and that her opponents so badly want to keep in the public eye — has new life.

The controversy erupted in March 2015, when the New York Times reported that Clinton had used a personal email account based on a private server in her Chappaqua, New York, home to conduct government business while she was secretary of state — raising the possibility that she had skirted rules that require employees to keep such documents as part of the State Department's official record.

By then, Clinton's staff had gone through tens of thousands of emails, weeding out those they thought needed to be turned over to the State Department, and deeming the rest personal.

Related: FBI Should Release Full Facts About Email Discovery

A few days later, Clinton — who had yet to formally announce her presidential candidacy — told reporters that she'd used the server out of "convenience." She added, "Looking back, it would have been better for me to use two separate phones, and two e-mail accounts. I thought using one device would be simpler, and, obviously, it hasn't worked out that way."

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That would turn out to be an understatement.

By July 2015, as signs of Clinton's apparent efforts to conceal the email system began to emerge, the inspectors general of the State Department and Office of the Director of National Intelligence asked the Department of Justice to get involved, to assess whether any classified information had been put in jeopardy.

The inspectors general said they'd taken a sample of 40 Clinton emails and found four with classified information that should "never have been transmitted via an unclassified personal system" — which seemed to contradict Clinton's earlier assertion that she hadn't emailed any classified material.

That triggered the FBI probe, focusing on the use of the server and not Clinton personally. Clinton's campaign announced that she had ordered her server turned over to the feds.

Related: Trump Commends FBI For 'Courage' To Investigate Additional Clinton Emails

The probe dogged Clinton for the entirety of the primary season and into the general election campaign, a controversy fueled by court-ordered batch-releases of the emails — more of which were later deemed to be classified retroactively — and a House committee's investigation of the 2012 terror attack on the U.S. Consulate in Benghazi, Libya.

At campaign events and under questioning by reporters, Clinton said she gave investigators all of her work-related emails, and insisted that she never sent anything marked as classified at the time via email. She apologized, but also began to accuse critics of using the case find ways to damage her campaign.

Throughout early 2016, some Clinton advisers and employees were interviewed by federal investigators. They included Bryan Pagliano, who set up the server. In July 2016, Clinton herself sat for a three-hour interview.

She told them, according to notes later released by the agency, that she did not know that "c" stood for confidential. The notes also revealed that Clinton apparently did not contemplate the potential problems that could result from going outside government servers, and did not understand what officials with top security clearances are supposed to do in such circumstances.

The notes also showed that the deletion of emails began after The New York Times first reported Clinton's use of a private email server. They showed that the FBI found no evidence that Clinton's email accounts or mobile devices were compromised, although "phishing" emails were sent to her private account on multiple occasions.

Of the 81 classified emailed chains sent on Clinton's private server, two were among the most sensitive because they are released only to a group of English-speaking countries who share a joint-intelligence sharing arrangement, the FBI notes revealed.

A few days after that interview, on July 5, Comey announced that the FBI had found no evidence that Clinton committed a crime. But Comey called Clinton and her staff "extremely careless in their handling of very sensitive, highly classified information."

Related: FBI Releases Notes From Interview With Hillary Clinton Over Emails

That sparked an uproar among Clinton critics, including her campaign rival, Donald Trump, who used it to argue that the election was rigged.

Comey defended his decision to a congressional panel and told lawmakers he did not expect to return to the investigation.

Then came Friday's bombshell.

Comey's announcement that the FBI would review a new batch of emails came less than two weeks before the Nov. 8 election, reinvigorating critics and putting Clinton, again, on the defensive.

The emails, federal officials told NBC News, were found during an unrelated inquiry into former U.S. Rep. Anthony Weiner, whose career was upended in a sexting scandal and whose wife, Huma Abedin, is a top Clinton aide.

Left unanswered is whether the emails, or the new controversy, will damage Clinton further.