Washington— Federal prosecutors investigating the possible mishandling of classified materials on Hillary Clinton’s private email server have begun the process of setting up interviews with some of her longtime and closest aides, according to two people familiar with the matter.

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 advisers, but a federal prosecutor in recent weeks has called their lawyers to alert them that he would soon do so, the sources said. Prosecutors also are expected to seek an interview with Clinton, though when remains unclear.

The interviews by FBI agents and prosecutors will help them understand whether Clinton or her aides knowingly or negligently discussed classified government secrets over a nonsecure email system when was secretary of state.

The meetings also are an indication that much of the investigators’ background work — recovering deleted emails, understanding how the server operated and determining whether it was breached — is nearing completion.

Many legal experts believe that Clinton faces little risk of being prosecuted for using the private email system to conduct official business, though that decision has raised questions among some about her judgment. They noted that using a private email system was not banned at the time, and others in government had used personal email for official business.

The bigger question is whether she or her aides distributed classified material in email systems outside of the State Department’s secure classified system. But even if prosecutors determine that she did, chances she will be found criminally liable are low. U.S. law makes it a crime for someone to knowingly or willfully retain classified information, handle it in a grossly negligent manner or to pass it to someone not entitled to see it.

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

Legally it doesn’t matter if the emails were marked classified or not, since government officials are obligated to recognize sensitive material and guard against its release. But legal experts noted that such labels would be helpful to prosecutors seeking to prove she knew the information was classified, a key element of the law.

“The facts of the case do not fit the law,” said Stephen Vladeck, a law professor at American University. “Reasonable folks may think that federal law ought to prohibit what Hillary did, but it’s just not clear to me that it currently does.”

Even so, her use of the private server, which was based at her home in New York, has become fodder for Clinton’s political opponents as she seeks the Democratic nomination for president.

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

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

Her attorney, David Kendall, declined to comment. Her campaign spokesman, Brian Fallon, said in an email that Clinton is ready to work with investigators to conclude the investigation.

“She first offered last August to meet and answer any questions they might have,” Fallon wrote. “She would welcome the opportunity to help them complete their work.”

Lawyers for her closest aides either did not respond to messages or declined to comment.