WASHINGTON — Federal prosecutors investigating the possible mishandling of classified materials on Hillary Clinton's private email server have begun the process of setting up formal interviews with some of her longtime and closest aides, according to two people familiar with the probe, an indication that the inquiry is moving into its final phases.

Those interviews and the final review of the case, however, could take weeks, all but guaranteeing that the investigation will continue to dog Clinton's presidential campaign through most, if not all, the remaining presidential primaries.

No dates have been set for questioning the advisers, but a federal prosecutor in recent weeks has called their lawyers to alert them that he would soon be doing so, the sources said. Prosecutors are also expected to seek an interview with Clinton, though the timing remains unclear.

“The interviews are critical to understand the volume of information they have accumulated,” said James McJunkin, former head of the FBI's Washington field office.

Many legal experts believe that Clinton faces little risk of being prosecuted for using the private email system to conduct official business when she served as secretary of state.

The bigger question is whether she or her aides distributed classified material in email systems that fell outside of the department's secure classified system.

Clinton has denied using the email account to send or receive materials marked classified. Though some of the emails have since been deemed to be too sensitive to release publicly, Clinton's campaign has attributed that to “over-classification run amok.”

“The facts of the case do not fit the law,” said Stephen Vladeck, a law professor at American University.

Even so, her use of the private server, which was based at her home in New York, has become fodder for Clinton's political foes.

Though Sen. Bernie Sanders has largely declined to use the email scandal against her in the Democratic primary race, Republicans have repeatedly said she should be indicted or disqualified from running for the nation's top office.

At a recent Democratic debate, Clinton grew exasperated when asked what she would do if indicted. “That's not going to happen,” she said.

Federal prosecutors granted immunity to one of Clinton's aides, Bryan Pagliano, who helped set up the server in Clinton's home. He has cooperated with the federal investigation and provided security logs that revealed no evidence of foreign hacking, according to a law enforcement official.

dwilber@tribpub.com