There was a FISA warrant on Manafort while he was in contact with DT.

The warrant extended "at least" into early January, so it must have been operative during the transition. This story came out in September but it's that much more interesting in light of recent events.

https://www.cnn.com/2017/09/18/politics/paul-manafort-government-wiretapped-fisa-russians/index.html

Washington (CNN)US investigators wiretapped former Trump campaign chairman Paul Manafort under secret court orders before and after the election, sources tell CNN, an extraordinary step involving a high-ranking campaign official now at the center of the Russia meddling probe.

The government snooping continued into early this year, including a period when Manafort was known to talk to President Donald Trump.

Some of the intelligence collected includes communications that sparked concerns among investigators that Manafort had encouraged the Russians to help with the campaign, according to three sources familiar with the investigation. Two of these sources, however, cautioned that the evidence is not conclusive.

SNIP

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.

SNIP

The FBI then restarted the surveillance after obtaining a new FISA warrant that extended at least into early this year.