The Justice Department granted immunity to the former State Department staffer who set up Hillary Clinton’s private email server at her home. Here’s what the FBI is looking to investigate and what it means for the Democratic presidential front-runner. (Victoria Walker/The Washington Post)

The revelation that the Justice Department has granted immunity to a former State Department staff member who worked on Hillary Clinton’s private email server is a likely indication that the investigation is nearing a conclusion, but should not be read as a sign that the leading Democratic presidential candidate is going to face criminal charges, legal experts said.

That Bryan Pagliano — a 2008 presidential campaign worker who set up the server in Clinton’s home — will avoid charges as he cooperates with FBI agents is a significant, if incremental, development, according to former federal prosecutors and white-collar defense lawyers who have been following the case.

It could mean that Pagliano, concerned about his legal exposure, might implicate others, including Clinton. But it also could be an indication that agents and prosecutors are winding down an inquiry that will not result in charges, said Justin Shur, a former deputy chief of the Justice Department’s Public Integrity Section who now works in private practice at the MoloLamken firm.

“I don’t know that I would necessarily jump to the conclusion that this person has ‘flipped,’ and now they’re going to say a bunch of incriminating things about other people,” Shur said, adding that the agents could simply be making sure they have spoken to everyone relevant to the investigation.

Brian Fallon, a spokesman for the presidential campaign, said in a statement that Clinton has been cooperating with the Justice Department and offered in August to meet with officials. He said the campaign was “pleased” that Pagliano, who invoked his Fifth Amendment rights before a congressional panel in September, was cooperating. It is unclear what level of immunity Pagliano received.

Sen. Charles E. Grassley (R-Iowa), chairman of the Judiciary Committee, and Sen. Ron Johnson (R-Wis.), chairman of the Homeland Security and Governmental Affairs Committee, sent a letter to the Justice Department Thursday requesting a copy of Pagliano’s immunity agreement, noting they had previously asked that any such arrangement include a provision requiring him to cooperate with their investigation.

Immunized witnesses can be critical. Prosecutors offered a generous immunity agreement to the businessman they said bribed former Virginia governor Robert F. McDonnell (R), and the businessman’s testimony at trial was pivotal to demonstrating the corrupt bargain between the two.

In Pagliano’s case, FBI agents and federal prosecutors probably want to know about the conversations he had with Clinton or her aides when he set up the server, said Matt Kaiser, a white-collar criminal-defense lawyer at Kaiser, LeGrand & Dillon.

“It’s an interesting development,” Kaiser said. “I think they’re going to push it as far as they can. They’re going to want to know everything they can know before they make a decision to charge somebody or not.”

The New York Times reported Thursday that Pagliano had turned over security logs to investigators, and those logs showed no evidence of foreign hacking.

The FBI is looking to wrap up the Clinton inquiry — a criminal investigation of the possible mishandling of classified information — in the coming months, according to a senior U.S. law enforcement official. There are no outward signs that prosecutors have convened a grand jury, a powerful tool that would allow them to subpoena witnesses.

The issue of Clinton’s use of a private email server was referred to the FBI in July after the Office of the Inspector General for the Intelligence Community determined that some emails on the server contained classified material. The State Department has said that 2,093 of Clinton’s emails released by the State Department contained some type of classified material, most at the lowest level of sensitivity, and that 22 additional emails were fully “top secret” and were not released.

Clinton and the State Department have said that none of the material was marked classified at the time it was sent, but Clinton has apologized and called her use of the private email account a “mistake.” Many in the legal community have said it would be difficult to imagine her being charged.

That is because the line between what is classified and what is not is “not inherently obvious,” and charging the former secretary of state would require prosecutors to prove that she knew what she was handling crossed that line, said Barry J. Pollack, a white-collar criminal-defense lawyer at Miller & Chevalier who defended convicted CIA leaker Jeffrey Sterling.

“If something has not been deemed classified, you’re asking a person to intuit how somebody else would make a subjective decision and hold that person responsible for the fact that they didn’t anticipate that somebody else might view the document as classified,” he said. “It’s almost a Rorschach test. Different people view it differently.”

Spokesmen at the FBI and the Justice Department have declined to discuss the investigation. Pagliano’s attorney, Mark J. MacDougall, also declined to comment on the immunity agreement.

Edward B. MacMahon Jr., a lawyer who represented Sterling and 9/11 conspirator Zacarias Moussaoui, said that although prosecutors probably could charge Clinton, politics may prevent them from doing so. Justice Department guidelines, he said, discourage prosecutors from bringing cases in such a way that they influence elections.

“Almost anybody else already would have been receiving a target letter,” MacMahon said. “Of that, there isn’t any question in my mind.”

Jacob Frenkel, a white-collar criminal-defense lawyer at the Shulman Rogers firm, said the immunity grant “raises exponentially the stakes in the investigation,” although because of the secrecy surrounding the investigation, it was hard to know where it might lead.

“This is a significant piece in providing clarity to an otherwise complicated jigsaw puzzle,” Frenkel said, “and we do not know exactly where Mr. Pagliano’s finger or fingers will point.”

Adam Goldman and Tom Hamburger contributed to this report.