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The investigation into Hillary Clinton's private email server is entering a new phase, as federal prosecutors are beginning to set up formal interviews with some of Clinton's closest aides and possibly even the Democratic presidential front-runner herself.

Judge Andrew Napolitano explained on "Fox and Friends" today that this is the last phase of the investigation, which comes after the 147 FBI agents who have been working on this case have gathered "a mountain of evidence."

Judge Napolitano said that they are now ready to test that evidence by asking questions of the five or so aides who are closest to Clinton.

He explained that each aide will have to decide if they will agree to be interviewed, as they are not obligated to do so.

"If they go in, they are not under oath, but it is very, very dangerous," the judge said. "If they mislead or lie to the FBI, that's a felony the equivalent of perjury, and they could be prosecuted."

He added that the aides and their lawyers don't know how much or how little the FBI actually knows, so it's even more precarious for them.

The judge said that the timing of these interviews - if they do happen - and the next steps in the investigation will be very important, because the Democrats need to know if their leading presidential candidate might be a criminal defendant in a felony prosecution.

"They should know that now," he said. "Prediction: Something will happen by May, which is a little over a month away."

Watch more above.

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