Of all the characters in the political drama of Hillary Clinton’s private email server, none has been more mysterious—and potentially more important—than a 40-year-old technology specialist named Bryan Pagliano.

Pagliano didn’t just set up the now infamous “homebrew” server in the basement of Clinton’s New York home, which she used for official business while serving as secretary of state. Pagliano has been the former secretary’s go-to IT guy for the past several years. He’s also the only person in the entire investigation of Clinton’s email who got an immunity deal, protecting him from possible criminal prosecution. That was Pagliano’s demand for telling FBI investigators about Clinton’s unorthodox system—a system that he apparently knows more about than anyone else.

Federal prosecutors only offer someone immunity when they think he has unique information that can’t be obtained anywhere else. And for his part, Pagliano’s lawyer has said that the fact that the Justice Department granted it shows that he had a reasonable “fear of prosecution.” Over what isn’t entirely clear. It could be because of his role in establishing and maintaining the server.

But Pagliano also had an unusual employment arrangement. He was pulling down a six-figure salary at the State Department, which put him at the high-end of the pay scale for what appeared to be an ordinary tech support job. But Paliano was also being paid on the side in cash by the Clinton family, something his immediate supervisors didn’t know. In fact, they were never clear on precisely what his job was and didn’t know that during office hours, Pagliano was working for Clinton personally to maintain her private email system.

What is clear is that Pagliano believed he could be in legal jeopardy as a result of his work for Clinton, and the FBI was willing to cut a deal.

It paid off. After speaking to him last December, FBI investigators trying to reconstruct Clinton’s system—and determine whether it broke any laws—had a better sense of its complexity. As it turns out, there was more than one server, and Clinton used multiple devices. When the email story broke last year, Clinton said she used a private server for “convenience,” so that she wouldn’t have to carry different devices for her personal and private accounts.

Congressional Republicans have seized on the FBI’s findings of multiple devices as evidence that Clinton is lying, and they have now asked the bureau to investigate whether she perjured herself in testimony last year that touched on the email system.

Pagliano appears to have been a linchpin in the technical aspects of the FBI investigation. Ultimately, Director James Comey recommended that the Justice Department not press charges against Clinton or her aides for mishandling classified information—secrets did pass through her private server, which Comey said may have been hacked by “hostile actors.” But in an extraordinary public statement this month, followed by lengthy congressional testimony, Comey said Clinton and her aides were “extremely careless,” and in detailing all the ways she’d risked national security or behaved recklessly, he handed Republicans plenty of political ammunition to use against her.

Pagliano has given no interviews to journalists. His lawyer, Mark MacDougall, declined to comment for this story.

As Clinton prepares to accept her party’s nomination for president in Philadelphia this week, the email scandal still haunts her. She’s the subject of two congressional investigations. The State Department is conducting an internal inquiry into how Clinton and her aides handled classified information. And a federal judge in Washington is weighing whether Clinton should be deposed under oath by a conservative watchdog group that has been one of the Clinton family’s tireless political foes.

Pagliano may well figure in these spinoff scandals. And he may find his own communications with his former boss laid bare. The Republican National Committee is in the midst of a lawsuit against the State Department to obtain Pagliano’s emails—which for reasons the department has never completely explained have been very hard to locate.

Congressional officials privately told The Daily Beast that they continue to be interested in hearing from Pagliano, under oath, about his work for Clinton. Two powerful senators have been hounding him for more than a year to testify and have said that now that he has an immunity deal, he should have nothing to fear from appearing before Congress.

But Pagliano has remained almost entirely silent in the face of his inquisitors. He has rebuffed congressional requests. When he was ordered to give a deposition to the conservative watchdog group, Judicial Watch, he declined to answer every question posed to him, invoking his Fifth Amendment right not to incriminate himself 125 times. The only statement he has given on the record was to the FBI, which has never released a transcript of the interview.

For Pagliano, working for Clinton was a major career booster, and personally enriching. But it has come at a cost. What started out as a dream job more than a decade ago has landed Pagliano a most unenviable role—a key witness in an election year scandal.

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Pagliano first came to work for Clinton in 2006, as part of her first presidential campaign, having worked as a systems engineer for a company that provides technical support and advice to nonprofits. With Clinton, he started out as a kind of assistant, “providing technical engineering and support,” but worked his way up to leading the campaign’s information technology operations, according to his LinkedIn profile. The two were friendly. On his Facebook page, Pagliano posted photos of him posing with the secretary, as well as her husband. They have since been removed.

Pagliano was responsible for the campaign headquarters’ data center, oversight of other technology staff in the field, and working with contractors. When Clinton accepted Barack Obama’s nomination to become secretary of state, Pagliano set up the server in the Clintons’ home in Chappaqua, New York. Bill Clinton had already been using a server for his emails, but it was deemed too small for the workload of a cabinet secretary.

Pagliano was paid, among other sources, by Clinton’s Senate leadership PAC, according to campaign finance records. A leadership PAC is used for expenses that can’t be paid out of campaign or committee funds. Clinton’s was set up in part to help fund other Democratic races. But an investigation by The Intercept found that money from the PAC was used more to benefit Clinton’s own campaign and her staff than other candidates.

Pagliano was well compensated. In the first four months of 2009—just before Pagliano took a job at the State Department working for the newly installed secretary—he was paid a total of $27,850 from the leadership PAC and two other campaign funds.

In May 2009, Pagliano was hired at the State Department, as a “Schedule C” employee, a political appointee. It’s easier to hire and fire such employees than it is career government workers, but they’re also subject to strict ethics rules.

Pagliano’s job came with a handsome salary—around $140,000 per year, according to personnel information compiled by FedSmith, an analysis company. That put him on the very high end of State Department earners. For example, Pagliano was making about $13,000 more than the highest base salary allowed for Foreign Service employees, which includes career diplomats who serve in overseas posts, sometimes dangerous ones.

Hiring Pagliano, a technology specialist, was itself unusual since the department is filled with similarly skilled personnel. But Schedule C employees also have a “confidential or policy-determining relationship to their supervisor and agency head,” according to the Office of Personnel Management. The agency head in this case was Clinton. Schedule C authorities let a cabinet official hire whomever she thinks is best suited for the job, even if that person doesn’t meet the on-paper requirements or is creating a redundant position.

Pagliano was also hired at the highest “grade,” 15, on the government pay scale. Career employees spend years climbing the pay ladder. Pagliano had no prior government service. And while Schedule C employees may earn higher salaries than their career counterparts—indeed, the authorities are sometimes used to attract highly paid, skilled workers from the private sector—Pagliano appears to have been exceptionally well compensated for someone with his background, which aside from working for the non-profit was limited to being Clinton’s technology director.

What exactly Pagliano did at the department, however, wasn’t clear to his bosses. And later, they would question whether his employment arrangement was above board. That’s because while earning that hefty salary as a State Department employee, Pagliano was also being paid to perform “technology services for the Clinton family,” Hillary Clinton’s lawyer told the State Department inspector general (PDF), which issued a blistering report in May on Clinton’s unorthodox use of a private email server—the one Pagliano installed and maintained for her while she was the secretary.

Between 2009 and 2013, Pagliano was paid “by check or wire transfer in varying amounts and various times,” the State IG found. He worked out of State Department headquarters but also made trips to New York to check on the server and maintain it.

Pagliano’s arrangement raised many questions for his direct supervisors at the department when it was revealed by the IG investigation. The State Department’s chief information officer and the deputy chief information officer—the top technology officials who oversaw Pagliano and wrote his performance evaluations—told investigators that during the four years Pagliano worked there, they didn’t even know he was working on Clinton’s email system. The impression at Foggy Bottom was that Pagliano had been brought on to support “mobile computing issues across the entire department.” His bosses thought he was at State to work for everyone, not exclusively for Clinton.

The officials told the IG that they “questioned whether [Pagliano] could support a private client during work hours, given his capacity as a full-time employee.”

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What’s more, Pagliano failed to list his outside income on a required personal financial disclosure that he filed each year, The Washington Post reported. Government personnel rules don’t prohibit a political appointee like Pagliano also earning a side income, but there are limits on how much he could earn, and the amounts must be disclosed. He would also have to report the income on his tax returns.

How much the Clintons paid Pagliano while he worked at the State Department is unclear. He declined to grant an interview to the State Department inspector general, as did Clinton and five of her top aides.

Neither his lawyer nor the FBI have said whether Pagliano’s immunity agreement covers his employment arrangement and any violations that could have occurred as a result of his collecting outside income or failing to report it. But immunity agreements can be fashioned to cover any manner of subjects.

The government gave Pagliano what’s known as “use” immunity, which means that anything he told the FBI in the course of its investigation of Clinton’s email system cannot be used to bring charges against him. (If evidence of a crime emerges from other sources, the government could still prosecute Pagliano.)

The full details of the immunity deal haven’t been revealed publicly. But some key aspects were revealed in a lawsuit brought by Judicial Watch, which is seeking information on another unusual employment arrangement—that of Huma Abedin, Clinton’s senior aide. She was allowed to hold multiple outside jobs, including for the Clinton Foundation, while also serving as Clinton’s deputy chief of staff at the State Department.

“The mere fact that the government was willing to offer Pagliano ‘use’ immunity here in exchange for his testimony indicates that his fear of prosecution is more than fanciful or speculative,” Pagliano’s lawyer, Mark MacDougall, wrote in a legal filing with the court hearing Judicial Watch’s case. The watchdog group also wanted to depose Pagliano. But his lawyer argued that would put him at risk.

“Mr. Pagliano’s prospective deposition will inevitably cover matters that might ‘furnish a link in the chain of evidence needed to prosecute,’” MacDougall wrote. “The Court has authorized Judicial Watch to obtain discovery relating to ‘the creation and operation of clintonemail.com for State Department business.”

That subject was also the focus of the FBI investigation. So, Pagliano had reason to believe that what he might say to Judicial Watch could put him at risk for prosecution, MacDougall argued.

As a result, Pagliano intended to assert his Fifth Amendment privilege and not answer any of Judicial Watch’s questions. The group didn’t try to force him. But they wanted to videotape the deposition. Pagliano would be captured on film, declining to answer dozens of questions about his old boss and her complicated, careless email system.

The judge ultimately ruled the deposition would be recorded. He also required Pagliano to hand over a copy of his immunity agreement, which was placed under seal.

Judicial Watch isn’t the only Clinton adversary that has locked on Pagliano and what he knows.

Earlier this month, members of the House Committee on Oversight and Government Reform questioned FBI Director James Comey about the findings of the bureau’s investigation.

Comey, who had by then already said that Clinton was “extremely careless,” left little doubt that Pagliano was a key witness.

“What about Bryan Pagliano?… Do you know if he knew that she [Clinton] was not following proper protocol here?” asked Rep. Buddy Carter, a Republican from Georgia, in regards to using a private email system, which the inspector general had determined was at odds with department rules.

“He helped set it up,” Comey replied.

“He helped set it up? So obviously he knew,” Carter said.

“Yeah. Obviously he knew that,” Comey said.

Comey, a former federal prosecutor, declined to discuss the specifics of Pagliano’s immunity deal, but he noted that in general, “You make a grant of immunity in order to get information that you don’t think you could get otherwise.” Comey said that as a prosecutor he had used immunity grants “many times.”

Two days earlier, Comey had explained publicly why he decided not to recommend a prosecution, and took the unusual step of describing how the investigation unfolded. While not naming Pagliano directly, Comey said that the FBI had spent “thousands of hours” figuring out the architecture of Clinton’s email system, which was far more complex than the public had realized.

“Piecing all of that back together—to gain as full an understanding as possible of the ways in which personal email was used for government work—has been a painstaking undertaking,” Comey said, made harder by the complex way in which the system was maintained.

“For example, when one of Secretary Clinton’s original personal servers was decommissioned in 2013, the email software was removed,” Comey said. “Doing that didn’t remove the email content, but it was like removing the frame from a huge finished jigsaw puzzle and dumping the pieces on the floor. The effect was that millions of email fragments end up unsorted in the server’s unused—or ‘slack’—space. We searched through all of it to see what was there, and what parts of the puzzle could be put back together.”

Clinton’s emails weren’t the only ones that have been hard to piece back together. Pagliano’s have also been difficult to find.

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In the many lawsuits brought under the Freedom of Information Act to force the release of Clinton’s emails and those of her aides—including one filed by The Daily Beast—Pagliano’s emails have been the hardest for State Department officials to locate.

Initially, the State Department claimed that there were no Pagliano emails—at least none that its investigators could discover.

A State Department official explained to The Daily Beast in May that the department had searched for copies of Pagliano’s emails in a backup known as a .pst file, but that officials couldn’t locate one for the period of time that covers Clinton’s tenure as secretary.

The Republican National Committee, which had filed a lawsuit seeking copies of Pagliano’s emails, was incredulous.

“It’s hard to believe that an IT staffer who set up Hillary Clinton’s reckless email server never sent or received a single work-related email in the four years he worked at the State Department,” Raj Shah, the deputy communications director for the RNC, told The Daily Beast at the time.

Also curious was that while the department found no .pst file for Pagliano’s work during Clinton’s tenure, officials did find one for his work as a contractor—after Clinton had left office.

In order to reconstruct Pagliano’s email record, the State Department looked for emails of people who were likely to have corresponded with him or about him. (One such message actually turned up in a batch of Clinton’s own emails, which have been released for months now on a rolling basis. Pagliano wrote to his boss to wish her a happy birthday. “To many more!” he wrote. Clinton forwarded the message to an aide with a request to “Pls respond.”)

In searching for emails like that, the State Department told a federal judge that it came up with approximately 392,000 documents totaling about 627,200 pages that were “potentially responsive” to the RNC’s request. So massive was the record, the department claimed, it would take 104 years to go through it and determine which emails were really ones to or from Pagliano.

Again, the RNC cried foul.

“It is simply not plausible that the United States Government lacks the technological capacity to determine how many of those emails were sent or received by Mr. Pagliano,” the committee responded in a legal filing.

But then, just this month, State came back with new information. Somehow, it had managed to narrow down that giant universe of emails to just 1,300 that were either to or from Pagliano or “cc’d” to him. The department was now confident that it could locate Pagliano’s emails and turn them over to the RNC.

What may appear to some to be a willful effort to keep Pagliano’s emails from the public could also be sheer incompetence in record keeping. The inspector general criticized the department’s archiving system, and department officials have acknowledged that they need to do a better job keeping track of officials’ emails.

But Republicans have seized on the missing emails as an indication of a possible coverup, meant to protect the Democratic nominee. “Such records might shed light on [Pagliano’s] role in setting up Clinton’s server, and why he was granted immunity by the FBI,” Shah told The Daily Beast. “But it seems that his emails were either destroyed or never turned over, adding yet another layer to the secrecy surrounding his role.”

Clinton’s political opponents aren’t the only ones to have questioned how State Department emails were preserved. Two technology employees told the inspector general that in late 2010 they “discussed their concerns about Secretary Clinton’s use of a personal email account” with John Bentel, who was then the director of Information Resource Management in the office of the Executive Secretariat, where Pagliano worked.

“In one meeting, one staff member raised concerns that information sent and received on Secretary Clinton’s account could contain Federal records that needed to be preserved in order to satisfy Federal recordkeeping requirements,” the IG found. “According to the staff member, the Director [Bentel] stated that the Secretary’s personal system had been reviewed and approved by Department legal staff and that the matter was not to be discussed any further.”

But that review didn’t happen. Judicial Watch now wants to depose Bentel under oath, too. The judge hearing the case, Emmet Sullivan, said this month that he thought the deposition should proceed. He’s expected to issue an order soon on that question, as well as whether Clinton herself should be deposed by Judicial Watch.

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Pagliano may not face prosecution over the email system, thanks to his immunity agreement. But he’s not necessarily free of the political scandal that has dogged his former boss for more than a year and that appears to be dragging down her poll numbers in key battleground states.

Sens. Chuck Grassley and Ron Johnson, respectively the powerful chairmen of the committees on the Judiciary and Homeland Security and Governmental Affairs, have been after Pagliano since last year to testify about the email system. Given that he has a immunity protection, the senators have questioned why he won’t speak.

The senators last contacted Pagliano in March, after multiple requests for his testimony. He has consistently declined the invitations and could assert his Fifth Amendment privilege if he were subpoenaed. Meanwhile, the RNC’s lawsuit should soon start producing Pagliano’s emails from when he worked at the State Department.

Refusing to answer questions doesn’t constitute any admission of guilt on Pagliano’s part. But his silence has only fanned the flames of intrigue surrounding his role in the email scandal and what more he may know about it.

To the extent that Clinton is still haunted by the controversy, Pagliano could get pulled back into it. For him, the biggest question of all may be, “How long can you stay quiet?”