A source close to the federal probe into Hillary Clinton's email scandal says one of the Democratic candidate's former IT specialists has proven to be a 'devastating witness'.

Bryan Pagliano, who worked as an IT specialist for Clinton when she was Secretary of State, sent shockwaves earlier this month when it was announced he struck an immunity deal with the FBI, agreeing to cooperate with the agency's investigation of Clinton's personal email server without the fear of being prosecuted.

A witness who is close to the investigation but who is not authorized to speak about it publicly told Fox News that Pagliano has been extremely helpful in unraveling the mystery surrounding Clinton's top-secret emails.

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A source close to the federal probe into Hillary Clinton's (right) email scandal says former IT specialist Bryan Pagliano (left) is proving to be a 'devastating witness'

'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,' the source said.

Essentially, Pagliano is able to tell investigators who had access, on what devices and when.

This has been helpful in placing where Clinton was when she accessed certain sensitive emails. Investigators are using pictures of Clinton on the road as Secretary of State, along with Pagliano's information, to find where they were accessed.

'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. '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.'

Clinton has fiercely denied claims that she sent or received emails that were marked as classified at the time.

But her distinction is misleading in the sense that classification is based on content and not markings - and not all classified documents are marked with that warning.

Therefore, Pagliano could hypothetically place where Clinton was when she read or responded to a 'classified' email, and investigators could illustrate it with a picture of Clinton holding whatever device she accessed the email, if such a picture exists.

The source also said that the investigation appears to be going well, judging by the behavior of the investigators.

Morale among the agents is 'very good and nobody is moping around, which is the first sign a big case is going south.'