Mike Pence and White House spokespeople have denied that the FBI's investigation into Russia had anything to do with Comey's firing — Comey was slated to testify in front of the Senate on Thursday. But under Comey, the FBI uncovered information about a number of connections between Trump associates and Russia. These are the 6 people from Trumpworld we know have had dealings with Russian officials.
What about the President? Other than the unverified dossier, which made claims about Trump's own personal contact with Russia, there has been no evidence of Trump directly colluding with Russia. The concern is the people he surrounded himself with during the campaign, and in some cases, in the White House.