(CNN) - CNN has learned that U.S. investigators wiretapped former Trump campaign chairman Paul Manafort.
It was authorized under secret court orders in 2014, long before Trump announced his candidacy. And it continued until early this year, after Trump was in the White House.
It's not known if President Trump was picked up on the surveillance.
Sources say the FBI investigation initially centered on work done by Washington consulting firms working for Ukraine's former ruling party.
At one point last year, the surveillance was stopped for lack of evidence, and was then restarted after a new warrant was obtained.
Sources say the second warrant was part of FBI efforts to investigate suspected ties between Trump campaign associates and possible Russian operatives.
Three sources say the communications sparked concerns that Manafort had encouraged Russians to help with the 2016 presidential campaign.
Two of those sources say the evidence is not conclusive.
Details of those communications have been given to special counsel Robert Mueller's team.
The White House declined to comment. A spokesperson for Manafort did not comment.
Manafort had earlier denied knowingly communicating with Russian intelligence operatives during the election.
Such warrants require the approval of top Justice Department and FBI officials.
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.
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