So Mueller made a brief statement today.
https://www.politico.com/story/2019/05/29/robert-mueller-statement-russia-investigation-text-transcript-1346453My impression is, the main reason for the statement was to get out of testifying before Congress. He said outright that he does not want to testify, and that he has nothing to say beyond what his report says -- that the report speaks for itself and he wants the report to be his last word on the matter. I don't know if this means that Congress will desist in its efforts to get him to testify, or if it means that he will ultimately refuse if they persist.
Another thing Mueller reiterated, which I've never understood, is that if they were certain Trump had committed no crime, they would have said so. Since when is it our system for an investigator to conclude that no crime was committed? Isn't this normally the job of judge or jury after crimes have been charged? This statement has seemed unfair to me since the first time Mueller made it.
There was one other point that I thought was quite finely parsed and so I expect the media to get it wrong. Mueller said that under Justice Department policy, a sitting President cannot be indicted. Mueller said that therefore, they concluded that they would not make a determination as to whether or not the President committed a crime. I think the media will report this as, 'Mueller says they would have indicted Trump if not for rule stating a President can't be indicted.' But that isn't what he said. He said they deliberately reached no conclusion about whether or not the President committed a crime.