Manafort was being wiretapped under a FISA warrant during the campaign:
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.
Good roundup of the implications here.