A grant of immunity to a former Hillary Clinton staffer confirms that the FBI is investigating the case as criminal matter and that the Justice Department is taking it seriously, according to a pair of former federal prosecutors.

The developments can only be taken as bad news for Clinton as she attempts to run for president while fending off persistent questions about her handling of emails on a private servers set up in her home. It could be the most important development in the case to date.

“It’s significant,” said Joseph diGenova, former U.S. attorney for the District of Columbia. “It’s confirmation that a criminal investigation is formally underway, not a security review as the Clinton campaign continues to maintain.”

Bryan Pagliano, who helped set up Clinton’s private email server, accepted the offer of immunity Thursday. DiGenova said Pagliano likely has knowledge that could implicate other Clinton aides and, perhaps, the former secretary of state, herself. An immunity deal also is an indication that this is no mere fishing expedition — to agree to offer Pagliano certain legal protections, investigators already have a good idea of what information he possesses.

“It’s not something that’s done idly … The don’t do a proffer letter unless there’s a serious criminal investigation,” said Andrew McCarthy, who was the lead prosecutor of defendants implicated in the first World Trade Center bombing in 1993. “It’s bad news for (Clinton) because it really indicates a serious investigation and that there’s some buy-in from the Justice Department.”

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McCarthy noted that the FBI could not, on its own, approve an immunity deal. The Justice Department would have to sign off.

There are two types of immunity, and it is not clear which one Pagliano received. One is statutory immunity, which would indicate that a grand jury already has been convened. In that process, a judge approves an immunity request by the Justice Department, and the witness — as long as he is truthful — cannot be prosecuted for his statements or leads deriving from them.

The second type of immunity is more limited and involves a “proffer letter” from a defense lawyer  outlining what information his client has. McCarthy said it sometimes is called “queen for a day immunity,” and it protects a witness from using his statements against him — but not leads those statements produce.

From Clinton’s point of view, “it doesn’t matter at this point,” DiGenova said. Either way, Pagliano is talking to the FBI, and if he reveals incriminating information, it could be damaging to others involved.

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Pagliano declined to testify in September before the House Select Committee on Benghazi, which uncovered the email scandal, citing his Fifth Amendment right against incrimination.

“That’s a pretty good sign he has information that is likely to put him in jeopardy and likely to put anyone in cahoots with him in jeopardy,” McCarthy said.

McCarthy said that in his experience, proffer immunity usually is given shortly before or even after criminal charges, as a prelude to a plea bargain. Often, cooperating witnesses who get immunity receive sentencing consideration but do not avoid prosecution, altogether. He also said that prosecutors usually do not grant immunity until they already have gathered a great deal of information.

The Clintons long have inspired intense loyalty from members of their inner circle. Susan McDougal, a figure in the Whitewater scandal during the administration of President Bill Clinton, refused to cooperate with the criminal investigation and spent 22 months in jail. Webster Hubbell, a former Justice Department official, spent 18 months in jail in connection to Whitewater. Although he agreed to testify, he frequently cited memory lapses on key issues.

DiGenova said non-cooperating witnesses are a concern.

“I would say that Cheryl Mills falls into this category,” he said, referring to Hillary’s counselor and chief of staff at the State Department.

DiGenova said further cooperation may not be needed, especially if the FBI succeeds in retrieving the 31,830 emails that Clinton ordered deleted. If those emails contain any information dealing her official capacity, that could be a crime. It also could be a crime if she obstructed justice or mishandled classified information.

“This is not a complex case,” he said. “This is a relatively simple case.”