The decision by Hillary Clinton to use a private email server as secretary of state has spawned an FBI investigation, multiple congressional inquiries and dozens of private lawsuits that demand copies of her messages. It's also become an issue in her bid for the Democratic presidential nomination.
Republicans on the campaign trail have raised the prospect that Clinton could be charged with a crime — even as she downplays the FBI probe and asserts she wants voters to be able to see all of her messages from that time.
In Thursday's debate in New Hampshire, Clinton said she is "100 percent confident" the email probe will not turn into a legal quagmire. "I have absolutely no concerns about it whatsoever," she told debate moderators on MSNBC.
"We've got to get to the bottom of what's going on here," Clinton added.
In that spirit, here's our attempt to set out the key questions and answers so far.
1. What is the FBI investigating?
The Justice Department and the FBI have not publicly confirmed the scope of their investigation. U.S. government sources tell NPR it involves possible mishandling of government secrets that appeared in Clinton's email messages while she worked as secretary of state.
The State Department said last month it would not release 22 emails, or 37 pages in total, from Clinton's private server at the request of the intelligence community because "they contain a category of top secret information." State Department spokesman John Kirby would not describe the substance of the emails.
But Sen. Dianne Feinstein, the top Democrat on the Intelligence Committee, and a political ally of Clinton, said that none of the email chains deemed top secret "originated with Secretary Clinton" and "it has never made sense to me that Secretary Clinton can be held responsible for email exchanges that originated with someone else."
2. What is Hillary Clinton saying about this?
In an interview with NPR's Morning Edition, Clinton told host David Greene she was "absolutely not" putting top secret information at risk with her private email setup. "I took the handling of classified materials very seriously," Clinton said. "The emails that I received were not marked classified. Now there are disagreements among agencies on what should have been perhaps classified retroactively, but at the time that doesn't change the fact that they were not marked classified."
Clinton's press secretary has said the real problem is a dispute among Obama administration agencies over what should be considered secret, a phenomenon he calls "overclassification run amok."
Her campaign team went on the offensive Thursday after NBC News reported that State Department investigators had found some secret material in the email accounts of former Secretary of State Colin Powell and of aides to former Secretary of State Condoleezza Rice. Powell told Politico that he had been interviewed by the FBI about it. But Powell also said he considered the issue an "absurdity" because the material in question involved harmless conversations among diplomats.
3. Could Clinton be charged with a crime?
The candidate appears to reject that proposition out of hand — and she cast the controversy as part of a long-running political effort to undo her.
"No absolutely not, I mean I can't prevent it from being a political case, which seems to be the motivation behind selective leaking and anonymous sourcing and the kinds of things that seem to be going on," Clinton told Morning Edition recently. "But it's like Benghazi, I'm just very concerned that the timing here is interesting and misinformation in concert with Republican members of Congress has been consistently leaked as we've gotten closer to Iowa and other primary elections, but it does not change the facts."
For months, Clinton has defended herself by arguing that none of the material on her server had been marked "classified," which lawyers say could help her advance the idea that she never intended to break the law. She did, however, sign paperwork at the State Department that said she understood that information could be classified even if it weren't marked that way. And if she did not originate top secret email chains, as her political allies suggest, that could mean the aides who sent her the material would be in more legal jeopardy than she.
Former Attorney General Michael Mukasey, who worked in the George W. Bush administration, recently wrote an opinion piece in the Wall Street Journal arguing that some form of a criminal charge, even if based on negligence, rather than intentional wrongdoing, is justified.
Clinton told a CNN interviewer last month that she had not yet been interviewed by FBI agents. That step typically comes near the end of an investigation. And U.S. sources tell NPR no recommendation from the FBI has been shared with the attorney general, who would make the final call on any such prosecution.
4. When will the FBI investigation end?
Federal authorities have offered no public timetable for their investigation. Typically, the Justice Department tries to avoid bringing charges too close to an election lest it influence the outcome of a political race.
FBI Director James Comey has promised lawmakers he will conduct the probe quickly and independently. "We don't give a rip about politics," Comey said. He also told Republican senators that the FBI was not briefing the White House about the status of the Clinton email investigation.
5. What happens next?
A federal judge in Washington, D.C., will hold a hearing Feb. 9, the same day as the New Hampshire primaries, on a Freedom of Information Act lawsuit by Vice News, seeking the release of Clinton emails. The State Department also has to turn over at least one more batch of her messages, a step that could come shortly before the Super Tuesday votes.