Attorney General William P. Barr leaves his house Monday in McLean. (Joshua Roberts/Reuters)

In his letter to Congress on Sunday, Attorney General William P. Barr said he and Deputy Attorney General Rod J. Rosenstein had concluded the evidence from special counsel Robert S. Mueller III’s report would not support criminal charges of obstruction of justice against President Trump. There’s no question the country needs to see Mueller’s full report and learn exactly what he found. But when Barr next testifies before Congress, lawmakers should also ask him to explain his legal reasoning on obstruction. The soundness of his conclusion might depend on how he got there.

Barr’s own views on obstruction as applied to the president are well documented. In June 2018, Barr wrote as a private citizen a controversial 19-page memo harshly criticizing the obstruction theory he believed Mueller was pursuing. The question now is whether Barr’s conclusion that there’s no case for obstruction was based primarily on the evidence uncovered by Mueller or on Barr’s own narrow interpretation of what the law requires.

The case for obstruction was never a slam-dunk. Consider, for example, the act most often identified as possible obstruction: Trump’s firing of James B. Comey as FBI director. Trump said in an interview with NBC’s Lester Holt that the “Russia thing” was on his mind when he fired Comey, leading many to claim that Trump had essentially admitted he had done it to thwart the Russia investigation.

But Trump could claim a lack of corrupt intent by arguing that he fired Comey not to impede the investigation but to put someone more competent in charge. Indeed, later in that same Holt interview, Trump said: “As far as I’m concerned, I want [the Russia investigation] to be absolutely done properly. ... [Comey’s] the wrong man for that position.”

A related defense would be lack of a “nexus,” which requires that an alleged obstructive act would, if successful, have the “natural and probable effect” of interfering with the due administration of justice. But firing the FBI director does not naturally impede any of the thousands of investigations pending at the FBI. We would expect ongoing investigations to continue under a new director — which is, of course, exactly what happened with the Trump investigation.

Depending on what else Mueller has found, a reasonable prosecutor could well look at these facts and conclude, at least regarding the Comey firing, that obstruction could not be proved beyond a reasonable doubt. If Barr’s decision was based on judgments such as these, it might be completely defensible. Was it?

Another example of alleged obstruction is Comey’s claim that Trump asked him to drop the investigation of former national security adviser Michael Flynn. In evaluating this action, Barr might have relied on the theory, detailed in his June 2018 memo and mentioned in his letter to Congress, that proof of the president’s corrupt intent is undermined by the lack of evidence of an underlying crime. In other words, the theory goes, why would Trump try to kill the Flynn investigation to keep Flynn from spilling the beans about a Russian conspiracy if there were, in fact, no beans to spill?

Barr correctly noted that the lack of an underlying conspiracy, “while not determinative,” bears on the president’s intent. But he fails to acknowledge there can be corrupt motives to cover up misconduct that is not criminal. For example, Flynn, Michael Cohen and others convicted of lying to investigators might have concealed contacts with Russia not because they were proof of a criminal conspiracy but because revealing them would have been politically devastating to the president. Trump could have sought to quash the Flynn case for the same corrupt reason. Did Barr adequately consider this possibility?

Barr should also have to contend with the perverse effects of his legal theory. Doesn’t it suggest that if you obstruct justice so thoroughly that your underlying crime can’t be proved, you are in the clear? Would that not encourage aggressive obstruction?

Most controversial of the legal theories in Barr’s June 2018 memo was his claim that a president could not be charged with obstruction for official executive actions, such as firing the FBI director, no matter how corrupt his motives. Trump’s lawyers made the same argument throughout the investigation. I and many others think that legal theory is wrong and places the president above the law. How important was that theory to Barr’s conclusion?

It is significant that, according to the letter, Rosenstein joined in Barr’s conclusions. Rosenstein has stood up to Trump and defended Mueller for nearly two years, is heading out the door and has no reason to protect the president. He is intimately familiar with this investigation, having overseen it from day one. His endorsement should carry considerable weight.

Nevertheless, Barr’s letter to Congress leaves a great many questions unanswered concerning the legal rationale for his conclusion on obstruction. Congress should insist that Barr answer them.

Read more:

E.J. Dionne Jr.: Six takeaways from Barr’s letter about Mueller’s probe

Jennifer Rubin: What Barr’s letter about the Mueller report says and doesn’t say

Harry Litman: Three puzzling aspects of Barr’s summary of the Mueller report

George Conway: Trump is guilty — of being unfit for office