Professor Jacobson at Legal Insurrection indicates that Comey's testimony suggests that Mueller is already going beyond the narrow mandate of Acting Attorney General Rosenstein and considering whether the president's discussion with him about General Flynn constitutes "obstruction."

By the Order from Deputy Attorney General Rod Rosenstein, Mueller includes within his jurisdiction “any matters that arose or may arise directly from the investigation.” Comey testified that he believes Mueller is evaluating the communications between Comey and Trump with regard to potential obstruction of justice. Indeed, Comey expressed certainty in his testimony that the Special Counsel was investigating Comey’s conversations with Trump:

COMEY: … I don’t think it’s for me to say whether the conversation I had with the president was an effort to obstruct. I took it as a very disturbing thing, very concerning, but that’s a conclusion I’m sure the special counsel will work towards to try and understand what the intention was there, and whether that’s an offense.

What started as concerns over Russian interference in the election now is about the interactions between Comey and Trump.

CBS News reported that Mueller reportedly gave approval for Comey to testify before Congress and that the testimony was coordinated. Comey testified that he was permitted to review his memos in preparation of his written opening statement for the Committee submitted the day before his live testimony:

COMEY: Yes. I think nearly all of my written recordings of my conversations, I had a chance to review them before filing my statement.

LANKFORD: Do you have a copy of any of the notes personally?

COMEY: I don’t. I turned them over to Bob Mueller’s investigators.

There are a lot of questions that need to be answered about how Rod Rosenstein came to appoint Mueller in those few days after the Comey leak, and whether Comey and Mueller, directly or indirectly, had any communications regarding Trump prior to Mueller’s appointment.

Regardless, we now have the prospect of the Special Counsel investigating and necessarily assigning credibility (or lack thereof) to witnesses, including Comey.