Mueller does seem to be going down the obstruction of justice road over Trump's firing of Comey. I'm sort of surprised. I mean yes, Trump fired him because he wouldn't end the Russiagate investigation of Flynn. BUT, presidents do have near bulletproof status in hiring and firing executive office folks. Nixon was not sunk for firing the entire dept of justice till he got to the Toady Bork who'd rid him of a meddlesome prosecutor
It may mean that Trump actually has a crime of Russian and Trump campaign collusion, and the crime is trying to cover it up.