CNN legal analyst Paul Callan has done some research into President Trump's claims about bias inside the Mueller investigation:
PAUL CALLAN, CNN: I started writing this piece about the whole situation that was raised by Grassley, and originally I was angered by President Trump's attacks on the FBI, I thought they were unjustified, but when I started to delve into the facts, boy there is some really disturbing stuff here.
What Grassley is talking about is this agent [Peter Strzok] who is a counterintelligence agent for the FBI. He is exchanging texts with his girlfriend Lisa Page which just trash the president...
They essentially, the two of them --the lawyer and the agent-- agree that the president is a threat, or would be a threat to the country if elected. Now he's in a central role in the Hillary email investigation, and as a matter of fact, Strzok was the person who went to James Comey right before Comey had that famous press conference where he kind of let Hillary off the hook and said the investigation was going to be dropped into criminal charges. Strzok was the guy who changed the language, which originally was that she was 'grossly negligent' in her handling of classified material, and he softened it to say 'extremely careless,' which is not criminal.
This is the same guy who the inspector general now has exchanging these emails showing that he has a personal bias against the president, so that is what Sen. Grassley wants a closer look at, and I think it is legitimate to look at the bias of the investigators.
Callan reiterated his comments in a column: Trump Is Right About The FBI