(Washington Post illustration; photo by Saul Loeb/AFP/Getty Images)

Robert Litt is a former general counsel for the director of national intelligence. Benjamin Wittes is editor in chief of Lawfare and a senior fellow at the Brookings Institution.

Republicans and Democrats alike have called for special counsel Robert S. Mueller III to testify before Congress. Given the determined resistance of the Trump administration to any congressional oversight, it is unclear when Mueller will testify.

Whenever it happens, there may be legitimate limits on Mueller’s testimony. For example, the law prevents him from disclosing matters occurring before the grand jury. And given Mueller’s well-deserved reputation for reticence and strict adherence to the law, he is unlikely to be willing to provide a political spin on his report. But Mueller’s reputation for probity — previously unquestioned by Republicans and Democrats alike — is all the more reason he should testify, and why that testimony should be public rather than behind closed doors.

Here are 20 questions we think Mueller would be able to answer when he does testify, and on which his answers would help the public understand the important issues his investigation and report raised:

1. The president has said that your report found “No collusion, no obstruction.” Did you make a determination that there was in fact no collusion between the Trump campaign and Russian efforts to influence the election?

2. Did you make a determination that there was, in fact, no obstruction of justice by President Trump?

3. When you say in your report that the evidence or the investigation “did not establish” something — as you said about conspiracy between the Trump campaign and the Russians — what does that phrase mean? Does it mean there is no evidence of such a thing?

4. Is it fair to say that you found substantial evidence of cooperation, shared objectives and contacts between Russian actors and the Trump campaign figures?

5. Is it fair to say that you found substantial evidence of eagerness on the part of figures associated with the Trump campaign, including the president himself, to obtain emails stolen from Hillary Clinton, including potentially from Russian hackers?

6. Is it fair to say that you found direct attempts by the Trump campaign, apparently including by the president himself, to coordinate with WikiLeaks concerning the release of emails stolen from the Democratic National Committee by Russian intelligence actors?

7. Did Attorney General William P. Barr discuss your report’s findings on obstruction of justice with you before making his own determination?

8. Do you agree with his determination that the facts would not support an obstruction-of-justice charge?

9. The attorney general claims that your decision not to determine whether the evidence on presidential obstruction constituted a crime did not flow simply from the Justice Department’s legal opinion that a sitting president is not amenable to criminal process. To what extent was this decision on your part a direct consequence of the Justice Department’s legal posture?

10. Was it your intention when you left that question open for the decision to be made by the attorney general? If not, were you deferring to future prosecutors after President Trump leaves office?

11. To what extent, if any, were you deferring to Congress and its impeachment authorities?

12. Your investigation was an outgrowth of an FBI counterintelligence investigation into Russian efforts to interfere in the 2016 election. What was the origin of that investigation? As a former federal prosecutor and FBI director, do you think there was an appropriate basis to open that investigation?

13. What role did the so-called Steele dossier play in your investigation? Under what circumstances, if any, do you think that politically motivated research should play a role in counterintelligence investigations?

14. In your investigation, did you find any evidence to suggest the existence of a campaign by Russia or any other foreign entity to feed disinformation to either you or to the FBI investigation that you took over?

15. In your investigation, did you become aware of any investigative step taken previously by the investigation you inherited that you did not believe was substantially merited or justified by the evidence?

16. To your knowledge, were any of the FBI’s investigative efforts directed in any way by the White House, in the Obama or Trump administrations?

17. The attorney general has talked of the FBI “spying” on the Trump campaign. Was any person who was then a member of the Trump campaign the target of any wiretapping? Are you aware of any efforts by the FBI to inquire into the Trump campaign’s political strategy?

18. The president has criticized your team for being a group of “angry Democrats.” Did you select members of your investigative team based on their political views? Do you think it is appropriate to inquire into the political views of career civil servants in such a matter?

19. How grave a threat to our nation does the Russian interference you uncovered represent?

20. What steps should the nation be taking to counter such interference in the future? Are you satisfied that the FBI and other government agencies are taking appropriate steps to thwart ongoing interference activities?

When Mueller does testify, he can expect far more questions than we could list here. But it is vital for the public’s understanding, and in order to preserve the security of U.S. elections in the future, that it hear from Mueller on these and other matters. Congress has an important fact-finding role to play, and special counsel Mueller has an important part to play in assisting the legislature, consistent with the rules governing prosecutors and investigations of this scope and significance.