With the FBI's noose closing around Hillary and her closest State Department cohorts as the Federal agency nears the end of its criminal probe, some are getting increasingly concerned about what they will and will not say on the record. One such person is Hillary's former State Department Chief of Staff, Cheryl Mills, who according to the WaPo walked out of an interview with federal investigators when an FBI official began to discuss a topic considered off-limits.

The off-limits questions reportedly concerned the way in which emails were given to the State Department to be distributed to the public. According to the Post, Mills worried that the questions would violate the attorney-client privilege, and investigators had previously agreed not to broach the subject. It is unclear when the interview occurred.

The Post adds that Mills and her lawyer left the room, though both returned a short time later. It is not completely uncommon for FBI agents and prosecutors to diverge on interview tactics and approach, and the people familiar with the matter said Mills answered investigators’ questions. Mills and her lawyer, Beth Wilkinson, also asked for breaks more than once to confer, the people said.

Investigators consider Mills to be a cooperative witness but the episode demonstrates the tension surrounding the criminal probe into possible mishandling of classified information involving the leading Democratic presidential candidate. In the coming weeks, prosecutors and FBI agents hope to be able to interview Clinton herself as they work to bring the case to a close.Cheryl Mills, Clinton's former State Department chief of staff, and her lawyer both returned to the interview room a short time later, according to the newspaper, citing several unidentified people.

As the Hill adds, the Tuesday afternoon report comes as the federal investigation related to Clinton’s exclusive use of a private email server throughout her time at the State Department appears to be coming to a close. Interviews of Mills and other top aides have reportedly been conducted in recent weeks, and Clinton herself is expected to answer investigators’ questions soon.

Still, the episode with Mills shows the process has not been entirely smooth Clinton and her top allies, who have repeatedly shrugged off concerns about the server. The Post reported that Mills was seen as a cooperative witness despite the brief walkout. Clinton, the likely Democratic presidential nominee, has said that the setup was a mistake made out of a desire for convenience and not a desire to circumvent federal recordkeeping or transparency laws.

In response to this story, Wilkinson said, “Ms. Mills has cooperated with the government.” The Clinton campaign also did not provide a response, but spokesman Brian Fallon has said repeatedly that Clinton is willing to answer investigators questions, and he added in a recent statement that “we hope and expect that anyone else who is asked would do the same.”

So far, investigators have no found evidence tying Clinton to criminal wrongdoing, though they are still probing the case aggressively. Charges have not been ruled out. In recent weeks, they have been interviewing Mills and other aides.

One former State Department staffer who worked on Hillary Clinton’s private email server, Bryan Pagliano, was granted immunity so he would cooperate as part of the probe. In a hilarious update, the State Department "admitted" on Sunday that it was unable to track down any emails between Pagliano and Clinton, and apologized for its incompetence, even though it is common knowledge that at least one email during the time period in question was sent out and has been captured.

There is no indication a grand jury has been convened in the case, although according to some this is largely due to alleged intervention on behalf of the DOJ which has been eager to quash the investigation since day one.