Hillary Clinton’s former IT specialist who was given immunity from prosecution in exchange for his testimony about setting up the former secretary of state’s private email server is turning out to be a “devastating witness,” sources say.

After striking his immunity deal, Bryan Pagliano has given the FBI a full range of details about how the unsecured Clinton server was set up, Fox News senior intelligence correspondent Catherine Herridge.

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

The source, who is not authorized to speak on the record due to the sensitivity of the ongoing investigation, said Pagliano has provided information allowing investigators to knit together the emails with other evidence, including images of Clinton on the road as secretary of state. The cross-referencing of evidence could help investigators pinpoint potential gaps in the email record. “Don’t forget all those photos with her using various devices and it is easy to track the whereabouts of her phone,” the source said.

The source went on to describe how important this information is to investigators.

“It is still boils down to a paper case. Did you email at this time from your home or elsewhere using this device? And here is a picture of you and your aides holding the devices.”

The intelligence source added that the FBI is “extremely focused” on the 22 emails classified “top secret” that went through her server.

“Mrs. Clinton sending them in this instance would show her intent much more than would receiving [them],” the source said. “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. Remember, NDI does not have to be classified.” According to the Congressional Research Service, NDI is broadly defined to include “information that they have reason to know could be used to harm the national security.”

Although a source close to Pagliano didn’t dispute the FBI’s basic claims, he minimized the IT specialist’s importance, calling him a “peripheral” witness who met with the bureau on a “limited basis.”

At last week’s Univision debate, Clinton refused to answer a question posed by moderator Jorge Ramos asking if she would suspend her campaign if an indictment results from the FBI’s criminal investigation.

“My goodness. That is not going to happen. I’m not even answering that question,” the Democratic presidential front-runner told Ramos at the Wednesday debate.

When it was first reported that Pagliano was offered immunity in exchange for his testimony, Clinton told CBS, “I’m delighted that [Pagliano] has agreed to cooperate, as everyone else has. And I think that we will be moving toward a resolution of this.”

Former Clinton co-chief of staff Huma Abedin once emailed a frequent Clinton companion that the former secretary of state was “often confused.” Her statement to CBS may have been one of those times.

Watch the clip, via Fox News.