Mystery solved? The Washington Post’s new tick-tock on the Robert Mueller investigation’s interactions with the White House might provide an answer to one of the puzzling questions left at the end. Why did Mueller decisively conclude that Donald Trump and his campaign didn’t collude with Russia but punt on obstruction?

Was it because Trump outplayed Mueller on an interrogation? After threatening a subpoena once early last year, Trump’s attorneys angrily objected to the idea and threatened to force Mueller into court. Mueller retreated and left the idea as an unspoken backstop while the White House prepared the ground by cooperating as fully as possible. The strategy was to build a case in court where they could claim that Mueller didn’t need to interrogate the president in order to get the answers to any questions:

Central to the Trump strategy — developed first by Cobb and Dowd and later carried out by Giuliani, Sekulow and the Raskins, as well as Flood, who from his White House perch represented the office of the presidency — was to cooperate fully with every request for documents and witnesses from Mueller, including Trump’s written answers to some questions.

Their goal: to satisfy Mueller’s hunt for information to the extent that the special counsel would not have legal standing to subpoena the president’s oral testimony. …

“No matter what question they would say they wanted to ask, I felt confident we could turn it over and say, ‘You already have the answer to it,’ ” Giuliani said. “If they said, ‘Why did you fire Comey?’ I’d give them five interviews, and particularly the Lester Holt tape, where he goes into great detail as to his reasons.”

Despite what some commentators have argued, the Supreme Court’s precedents on executive privilege and subpoenas have only related to documents, not testimony from a president. That was also the case in the prosecution of Mike Espy, the former Agriculture Secretary for Bill Clinton, and also Clinton himself, who voluntarily testified to a grand jury … much to his regret. (Ken Starr withdrew the subpoena in an agreement to avoid a court fight.) No one was sure how the Supreme Court would rule on forcing a president to testify to a grand jury, and Giuliani told the Post that he wasn’t sure the Trump team would prevail if it came down to it.

It never did, however, as Mueller never tried issuing a subpoena. Instead, after the one threat, they kept requesting a voluntary sit-down. Even after Trump provided written answers to a questionnaire, the special counsel team insisted it needed to talk with Trump, and specifically about obstruction. However, they never presented any evidence-based justification for such an interrogation, which led Trump’s team to conclude that it was a fishing expedition at best, and a perjury trap at worst:

Mueller’s team kept insisting it needed to interview the president — but never followed through with an actual demand.

Mueller and Quarles would stress that they needed to know Trump’s intentions when he fired Comey and took other actions that could have thwarted the Russia investigation. Jane Raskin would respond by pressing them for a legal justification for seeking to interview the president, according to a person familiar with the negotiations.

The president’s team asked, “What evidence have you obtained that justifies you interviewing the president?” according the person, who added that Mueller’s office was “never able to articulate a compelling case. They never gave up asking, but they had no good answer for that question.”

They tried in December and January to get Trump to talk with them, to no avail. Instead, Mueller closed up shop and left the obstruction question unanswered. He left it to Barr and Rosenstein to reach the final conclusion on obstruction, which had critics and observers from James Comey to Andrew McCarthy asking the same question: If Barr and Rosenstein could answer that question, why did we need a special counsel at all? 

After reading through the Post’s report, the answer might be that Mueller left it open to take one last parting shot at Trump and his attorneys. If Trump never answered questions on obstruction, the explanation might go, then how can we know whether he committed obstruction of justice? Barr and Rosenstein dealt with that rather straightforwardly by noting that no evidence of obstructive intent had been developed — and since no evidence of an underlying crime had emerged, what was there to obstruct?

Mueller could have easily reached the same conclusion, but for some strange reason chose not to do so. Perhaps that was from a scrupulous adherence to the literal, or perhaps Mueller and his team wanted a little payback for having been outplayed on a presidential interrogation.