Attorney General William Barr's letter Sunday to congressional leaders is designed to provide the "principal conclusions" of special counsel Robert Mueller 's probe into Russian interference in the 2016 presidential campaign. But on one of the two central topics from the Mueller report - possible obstruction of justice by President Donald Trump - Mr Barr's letter leaves unanswered more principal questions than it answers.

The first puzzling aspect of the Barr letter is its report that Mr Mueller, after making a thorough factual investigation of evidence bearing on possible obstruction of justice, "determined not to make a traditional prosecutorial judgment." Mr Mueller's report, we are told, is rather perfectly fence-straddling, stating only that "while this report does not conclude that the president committed a crime, it also does not exonerate him."

So Question No. 1: Why did Mr Mueller, whose charges as a prosecutor include making precisely these sorts of judgments and bringing them to a federal grand jury, decline to do so in this case? Obviously, he was able to reach decisions with respect to the 37 individuals or companies that he charged over the course of his investigation. What extra-prosecutorial considerations caused the famously dutiful and thorough Mr Mueller to leave such a core part of his job unfinished?

Mr Barr's letter then informs Congress that he and Deputy Attorney General Rod Rosenstein decided to step into the breach. Mr Barr writes that Mr Mueller's decision "to describe the facts of his obstruction investigation without reaching any legal conclusions leaves it to the Attorney General to determine whether the conduct described in the report constitutes a crime." But this cryptic statement is really just a description of the gap, not an explanation for why Mr Barr stepped in.

That raises the second critical question: Did Mr Mueller ask Mr Barr to step in? Or is Mr Barr simply asserting a general power to reach conclusions for the Justice Department that Mr Mueller thought he himself couldn't or shouldn't? While in some bureaucratic sense, the attorney general, as head of the Justice Department, bears responsibility for all of the department's decisions, I am unaware of a single instance in my years in the Justice Department in which a final prosecutorial decision was left to the attorney general without so much as a recommendation from the actual prosecutor.

Mueller investigation: The key figures Show all 12 1 /12 Mueller investigation: The key figures Mueller investigation: The key figures Robert Mueller is the special counsel overseeing the investigation into Russia's meddling in the 2016 election, and potential obstruction of justice by the president. Mr Mueller has a pristine reputation in Washington, where he was previously in charge of the FBI. Throughout his investigation, he and his team have been notoriously tight lipped about what they know and where their investigation has led. REUTERS Mueller investigation: The key figures Former FBI director James Comey was the catalyst that led to the appointment of special counsel Robert Mueller. Mr Comey was fired by the president after Mr Trump reportedly asked him to drop his own Russia investigation. Mr Trump has long maintained that the investigation is a "witch hunt". AFP/Getty Images Mueller investigation: The key figures Deputy attorney general Rod Rosenstein had authority over the special counsel investigation for much of the two years it has been active. Mr Rosenstein found himself with that responsibility after then-attorney general Jeff Sessions recused himself from that oversight. AP Mueller investigation: The key figures Attorney general Jeff Sessions's decision to recuse himself from oversight of the special counsel investigation may have cost him his job in the end. Mr Sessions resigned last year, after weathering a contentious relationship with Donald Trump who vocally criticised his attorney general for taking a step back. Mr Sessions recused himself from the oversight citing longstanding Justice Department rules to not be involved in investigations overseeing campaigns that officials were apart of. AP Mueller investigation: The key figures Attorney General William Barr is currently responsible for oversight of the special counsel investigation. Mr Barr's office will be the first to receive the Mueller report when it is finished. His office will then determine what portion or version of that report should be delivered to Congress, and also made public. EPA Mueller investigation: The key figures Michal Cohn is the president's former personal lawyer, who has been helping the special counsel investigation as a part of a plea deal over financial crimes, and campaign finance crimes, he has pleaded guilty to. Among those crimes, Cohen admitted to facilitating $130,000 in hush money payments to adult film actress Stormy Daniels during the 2016 campaign. Cohen has said he did so at the direction of Mr Trump. Cohen has also admitted that he maintained contacts with Russian officials about a potential Trump real estate project in Moscow for months longer than Mr Trump and others admitted. The talks continued well into 2016 during the campaign, he has said. AP Mueller investigation: The key figures Stormy Daniels has alleged that she had an affair with Donald Trump in 2006, soon after Melania Trump gave birth to Baron Trump. The accusation is of particular importance as a result of the $130,000 hush money payment she received to keep quiet about the affair during the 2016 campaign. AP Mueller investigation: The key figures Paul Manafort was Donald Trump's former campaign chairman. Manafort was charged alongside Rick Gates for a slew of financial crimes, and was convicted on several counts in a Virginia court. He then pleaded guilty to separate charges filed in a Washington court. Manafort has been sentenced to just 7.5 years in prison for his crimes — in spite of recommendations from the special counsel's office for a much harsher sentence. AP Mueller investigation: The key figures George Papadopoulos was one of the first individuals associated with the Trump campaign to be charged by the Mueller probe. He ultimately received a 14 day prison sentence for lying to investigators about contacts he had with Russian officials. AP Mueller investigation: The key figures Roger Stone is a well known political fixer and operative, who has made a name for himself for some dirty tactics. He has been charged by the Mueller probe earlier this year, and he has been said to have had prior knowledge that WikiLeaks planned on publishing stolen emails from the Hillary Clinton campaign in 2016. Getty Images Mueller investigation: The key figures Rick Gates was charged alongside former Trump campaign chairman Paul Manafort for a range of crimes. Gates, who worked alongside Manafort for a pro-Russia Ukrainian political party. The two were charged with conspiracy and financial crimes. Gates pleaded guilty. AP Mueller investigation: The key figures Former national security adviser Michael Flynn was one of the first casualties of the Russia scandal, and was forced out of his position in the White House weeks after Donald Trump took office. Flynn pleaded guilty in 2017 to "willfully" making fraudulent statements about contacts he had with Russian officials including former Russian ambassador to the US Sergey Kislyak. Flynn then lied to Vice President Mike Pence about that contact. REUTERS

We need to know the answer. If, say, Mr Mueller's reason for refusing to exercise this judgment was that he believed the involvement of the president made the question a political one for Congress, Mr Barr's move would represent a rank overruling of a key conclusion of Mueller, as well as a power grab from Congress.

The final unanswered question, and perhaps the most consequential: What was the nature of the analysis that Mr Barr and Mr Rosenstein applied in deciding that Mr Mueller's evidence was not sufficient to establish that the president committed obstruction? The consensus of many scholars and commentators, based just on the publicly available evidence, has been that the case for obstruction was strong. Did some additional confidential evidence sway Mr Barr and Mr Rosenstein? Was it some particular legal reading of the obstruction statute?

President Trump claims 'total exoneration' in Mueller report

This last possibility is unsettling. Mr Barr's letter says that he and Mr Rosenstein consulted with the department's Office of Legal Counsel before coming to their conclusion. This raises the possibility that the Mr Barr analysis is premised on some controversial and expansive view of executive power that neither Congress nor the courts would endorse.

We know that Mr Barr wrote an unsolicited letter to department officials before he took office that could be read to embrace the view that the president cannot obstruct justice while exercising his enumerated powers, such as the pardon power, regardless of his motive for doing so. It would be an outrage if this were the basis for Mr Barr's and Mr Rosenstein's decision. That is not simply because the view is thoroughly wrong and discredited - which it is - but because it would effectively pre-empt, or sharply hamstring, the ability of both coordinate branches to decide the question.