The immunity deal granted to a former Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE staffer indicates the FBI’s investigation into her email practices is accelerating, former prosecutors say.

Clinton was riding high on Tuesday following a series of decisive primary victories that seemed to put her on an unstoppable path to the Democratic presidential nomination.

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But after her campaign celebrated big victories against Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.), the marquee issue that Republicans hope will sink her in the general election was thrust back into the spotlight.

The day after her Super Tuesday victories, a law enforcement official leaked that Bryan Pagliano, who helped set up Clinton’s personal server, has been granted immunity in exchange for his cooperation with the FBI’s investigation.

Pagliano, a former information technology official with the State Department, is seen as a key figure in the investigation into whether Clinton illegally transmitted classified information over her private server.

Clinton has consistently maintained no information that was classified at the time passed through her server, and a spokesperson immediately said that the campaign was “pleased” that Pagliano is cooperating with the FBI’s investigation.

Still, the immunity deal adds to the appearance of possible misconduct in Clinton’s email practices — and creates more uncertainty for her campaign.

Republicans have repeatedly invoked the possibility of criminal charges against Clinton in the server investigation. GOP front-runner Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE has suggested Clinton might not be able to run for president, while Sen. Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE (R-Texas) on Wednesday called the immunity deal “an ominous development for the Clinton campaign.”

“This is nothing but bad news for Hillary,” said Matt Mackowiak, a Republican strategist.

Deepening Republican suspicions, Pagliano has previously declined to speak to investigators from both the Statement Department Office of Inspector General and the FBI. In September, he exercised his Fifth Amendment rights in refusing to provide answers to the House Select Committee on Benghazi, citing the "current political environment.”

Legal experts say the kind of deal the Department of Justice (DOJ) has struck with Pagliano is common in cases where investigators want to work their way up the food chain in uncovering possible criminal activity.

Still, several former DOJ prosecutors cautioned that the granting of immunity in no way indicates that charges against Clinton are likely.

“If prosecutors confer immunity in a case, in general it means that they at least have some intention of pursuing the matter to a grand jury. Whether they ultimately decide to pull the trigger and press charges is a whole other question,” said Bob Ray, a former federal prosecutor who was head of the Office of the Independent Counsel during the Whitewater investigation of Bill and Hillary Clinton, and current co-chair of Fox Rothschild LLP’s white collar compliance & defense practice.

What the immunity deal does suggest, experts say, is twofold.

First, that investigators believe Pagliano has potentially valuable information that will help them determine whether any laws were broken.

Second, that the Department of Justice is at minimum setting the stage for the possibility of a criminal case.

“It doesn’t necessarily signal that there will be charges. What it does signal is that they are accelerating past the investigation stage and things have now risen to the level of a grand jury investigation,” Ray said.

Legal experts note that the Department of Justice (DOJ) — under whose authority the FBI operates — doesn’t have the power to grant immunity. Federal judges are the only ones with that power, and only at the request of a DOJ prosecutor.

But for a DOJ prosecutor to make such a request is “a pretty significant step,” Ray argued, one that prosecutors don’t typically take unless they believe the testimony is worth letting go of a smaller fish.

“Prosecutors are in the business of making cases — you don’t hand out immunity unless the hope is it gets you additional evidence that you would not be able to get, and that [the] evidence leads towards being able to perfect a charge,” he said.

Security experts say Pagliano could know a great deal.

“I think of him as Sammy ‘The Bull’ Gravano; he knows where the bodies are buried and he could bring down the whole organization,” said Morgan Wright, a cybersecurity consultant who has worked with tech companies like Cisco and Alcatel-Lucent, referencing the underling who helped bring down mob boss John Gotti.

According to reports, Pagliano was the central figure behind Clinton’s server configuration.

“There was no crew,” Wright said. “This was a one-man shop.”

As a result, Pagliano would have the “most direct knowledge” of how the server was set up, said Peter Toren, a cyber crime attorney and former DOJ computer crimes prosecutor.

He likely can provide pivotal details, including what security measures — such as encryption — were put in place when the server was installed, and whether backups were made.

Perhaps more importantly, Pagliano might have been part of conversations regarding what Clinton’s camp said the server would be used for, and how the team wanted to manage confidential information that could pass through the device.

“Was he told, ‘Oh this server is going to have confidential, or top secret, or secret State Department communications on it?’” Toren said.

All of the conversations with Clinton’s staff at State could give investigators more targets and leads.

“This guy’s low-hanging fruit,” Wright said. “But he’s going to lead them to bigger things.”

Clinton supporters have tried to downplay the significance of Pagliano’s testimony.

“I don't sense any anxiety over what he knows or what he would say," a longtime Clinton adviser said. “I believe the Clinton people were open to him speaking to the Benghazi committee, but his lawyer and he felt otherwise, apparently.”

Amie Parnes contributed