A secret order authorized by the court that handles the Foreign Intelligence Surveillance Act (FISA) began after Manafort became the subject of an FBI investigation that began in 2014. It centered on work done by a group of Washington consulting firms for Ukraine's former ruling party, the sources told CNN.
The surveillance was discontinued at some point last year for lack of evidence, according to one of the sources.
The FBI then restarted the surveillance after obtaining a new FISA warrant that extended at least into early this year.
Sources say the second warrant was part of the FBI's efforts to investigate ties between Trump campaign associates and suspected Russian operatives. Such warrants require the approval of top Justice Department and FBI officials, and the FBI must provide the court with information showing suspicion that the subject of the warrant may be acting as an agent of a foreign power.
The conversations between Manafort and Trump continued after the President took office, long after the FBI investigation into Manafort was publicly known, the sources told CNN. They went on until lawyers for the President and Manafort insisted that they stop, according to the sources.
The FBI also eavesdropped on Carter Page, a campaign associate that then candidate Trump once identified as a national security adviser.
http://www.cnn.com/2017/09/18/politics/paul-manafort-government-wiretapped-fisa-russians/index.html