The source said Pagliano told the FBI who had access to the former secretary of state's system – as well as when – and what devices were used, amounting to a roadmap for investigators.

"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 intelligence source said.

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.