GOP Sen. Tom Cotton had a burning question for James Comey:

“Do you believe Donald Trump colluded with Russia?”

Comey had an answer Cotton didn’t exactly like:

 “It’s a question I don’t think I should answer in open setting.”

Comey said that the FBI didn’t have an open investigation into Trump when he left the agency but added, “That's a question that will be answered by the investigation, I think.”
That potentially signals that special counsel Robert Mueller is investigating possible collusion between Trump and Russia.
Cotton immediately pivoted to statements by Sen. Dianne Feinstein (D-CA) that she wasn’t aware of evidence that Trump colluded with Russia. Just guessing that both Comey and Mueller know a little more about possible Trump-Russia collusion than Feinstein does, and Comey doesn’t want to corrupt that investigation in any way.