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 still take many weeks, all but guaranteeing that the investigation will continue to dog Clinton's presidential campaign through most, if not all, of the remaining presidential primaries.

No dates have been set for questioning the advisors, 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 also are expected to seek an interview with Clinton herself, though the timing remains unclear.