The FBI obtained a secret court order last summer to monitor the communications of an adviser to presidential candidate Donald Trump, part of an investigation into possible links between Russia and the campaign, law enforcement and other U.S. officials said.
The FBI and the Justice Department obtained the warrant targeting Carter Page’s communications after convincing a Foreign Intelligence Surveillance Court judge that there was probable cause to believe Page was acting as an agent of a foreign power, in this case Russia, according to the officials.
This is the clearest evidence so far that the FBI had reason to believe during the 2016 presidential campaign that a Trump campaign adviser was in touch with Russian agents. Such contacts are now at the center of an investigation into whether the campaign coordinated with the Russian government to swing the election in Trump’s favor.
During the Republican primary season last year, Trump identified Page as a foreign policy adviser to his campaign.
It's worth noting that a FISA warrant is not a simple thing to obtain. FBI Director James Comes testified that application for FISA warrants are thicker than his wrists, referencing the amount of work that goes into compiling evidence for the warrant.
Much more at the link - I recommend a full read.