Law enforcement and intelligence officials say the FBI was able to obtain a secret court order that allowed them to monitor the communications sent from former Trump adviser Carter Page.
The order, that was acquired last summer, was used as part of the investigation into the alleged links between Russia and the Trump campaign.
The FBI and Justice Department were able to convince a Foreign Intelligence Surveillance Court judge there was probable cause to think that Page was acting as Russian foreign power agent.
FBI Director James Comey says the applications for such warrants, called Foreign Intelligence Surveillance Act (FISA), are massive and that in order to get them, a lengthy declaration has to be made to prove their need.
The application for the intelligence on Page laid out the investigator's beliefs that he was a Russian government agent and knowingly engaged in activities on their behalf.
The FISA warrant also cites numerous occasions when Page had made contact with Russian intelligence operatives and allegations that Page met with a Putin confidant
Page has yet to be formally accused of any crimes and it isn't clear if the Justice Department will later seek charges against him.
In an interview Page denies any wrongdoing within the campaign or with having those kind of ties to Russia.
“This confirms all of my suspicions about unjustified, politically motivated government surveillance...I have nothing to hide.”
The White House, the FBI and Justice Department have yet to comment.