Bryan Pagliano, the former State Department computer network technician who managed Hillary Clinton’s email system, is being described as a “devastating witness” for the FBI in its investigation into whether classified information was mishandled on the Democratic presidential candidate’s email server.

According to Fox News, Pagliano has provided information that will allow investigators to piece together who had access to Clinton’s server and when.

“Bryan Pagliano is a devastating witness and, as the webmaster, knows exactly who had access to [Clinton’s] computer and devices at specific times,” an intelligence source close to the case told Fox News. “His importance to this case cannot be over-emphasized.”

The FBI granted Pagliano immunity late last year in exchange for his cooperation with the investigation. He has not spoken publicly about his involvement in the case, but Clinton has said that she is glad that her former server maintenance man is talking to the FBI.

A staffer on Clinton’s 2008 presidential campaign, Pagliano invoked his Fifth Amendment right against self-incrimination during a meeting with the House Select Committee on Benghazi in September.

Information provided by Pagliano could also help investigators determine whether there are any gaps in Clinton’s email records. The former secretary of state, who says she paid Pagliano for his services out of her own pocket, has claimed that she turned all of her work-related emails over to the State Department in December 2014. But that claim has come into question since it was revealed in June that Clinton failed to turn over at least 15 emails she exchanged with her friend Sidney Blumenthal.

Clinton has not said whether she deleted other work-related emails while she was in office.

“Don’t forget all those photos with her using various devices and it is easy to track the whereabouts of her phone,” Fox’s source said.

According to the source, investigators are focused on two main areas in the investigation: the 22 “Top Secret” emails that were found on Clinton’s server and the reason for the creation of the off-the-books email system.

“Mrs. Clinton sending them in this instance would show her intent much more than would receiving [them],” said the intelligence source.

“Hillary Clinton was at a minimum grossly negligent in her handling of NDI [National Defense Information] materials merely by her insisting that she utilize a private server versus a [U.S. government] server.”

More than 2,000 emails out of the 52,000-plus pages of records Clinton gave to the State Department contain information that has been classified retroactively. And of the 22 “Top Secret” emails, which Clinton was likely forwarded from her aides, at least one contains information that was classified when it was sent.

Clinton has denied that she is the focus of the FBI’s investigation. She also grew visibly angry during a Democratic debate on Wednesday when moderator Jorge Ramos asked her if she will drop out of the presidential race if she is indicted. (RELATED: Hillary Gets Mad When Asked If She’ll Quit Race If She’s Indicted)

“Oh, for goodness, that’s not going to happen,” Clinton said. “I’m not even answering that question,”

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