Trump had grown angry with the Russia investigation — particularly Comey admitting in front of the Senate that the FBI was investigating his campaign — and that the FBI director wouldn’t support his claims that President Barack Obama had tapped his phones in Trump Tower.

…In his letter dismissing Comey, Trump said the FBI director had given him three private assurances that he wasn’t under investigation. The White House declined to say when those conversations happened — or why Comey would volunteer such information. It is not the first time Trump has publicly commented on an ongoing investigation — typically a no-no for presidents. He said earlier this month that Comey had done Clinton a favor by letting her off easy.

Behind Comey’s firing: An enraged Trump, fuming about Russia – POLITICO

hmmmm