Alex Wong/Getty Images

Sen. Kamala Harris questioned Attorney General William Barr about Deputy Attorney General Rod Rosenstein's dual roles as both witness in and supervisor of the Robert Mueller investigation.

First, some background: Mueller's report disclosed a previously unknown episode in which Rosenstein resisted Trump's efforts to get him to take the blame for the firing of FBI Director James Comey.

That event was examined as a key piece of Mueller's investigation of Trump's possible obstruction of justice — meaning Rosenstein was an important witness in the probe. Rosenstein also oversaw the probe beginning in early 2017.

Here's how the exchange between Harris and Barr played out:

Harris: Did the ethics officials in your office in the Department of Justice review the appropriateness of Rod Rosenstein being a part of making the charging decision on an investigation which he is also a witness in?

Barr: So, as I said, my understanding was he had been cleared and he had been cleared before I arrived.

Harris: In making a decision on the Mueller report?

Barr: Yes.

Harris: And the findings of whether or not the case would be charged on obstruction of justice? He had been cleared on that?

Barr: He was the acting attorney general on the Mueller investigation.

Harris: Had he been cleared to make —

Barr: I'm informed that before I arrived, he had been cleared by the ethics officials.

Harris: Of what?

Barr: Of serving as acting attorney general on the Mueller case.

Harris: How about making a charging decision on obstruction of justice, the underlying offices which include him as a witness?

Barr: That is what the acting attorney general's job is.

Harris: To be a witness and to make the decision about being prosecuted?

Barr: Well, no. But to make charging decisions.

Harris: I have nothing else. My time has run out.