On coordination with the Russians, special counsel Robert Mueller was clear, according to the attorney general's letter to Congress Sunday: He did not find "that the Trump campaign or anyone associated with it conspired or coordinated with Russia in its efforts to influence the 2016 U.S. presidential election."

But on the question of obstruction of justice, Mueller, a Vietnam combat veteran who led the FBI after 9/11, did something that surprised many who know him. He punted.

He left the question of whether there was sufficient evidence that President Donald Trump committed the crime of obstructing justice to the attorney general, William Barr — a man who criticized the special counsel's investigation before taking office.

Mueller didn't draw a conclusion one way or the other on obstruction, Barr wrote, but instead set out evidence on both sides of the question. Barr said that was because Mueller considered those facts and legal questions difficult to answer.

Based on that, Barr, along with Deputy Attorney General Rod Rosenstein, decided that the evidence collected by Mueller did not establish that Trump committed the crime of obstruction of justice.

One expert noted how curious it was that Mueller didn't step into the obstruction issue.

"The whole point of the special counsel was to (mostly) take the investigative and prosecutorial decisions away from the Trump appointees," said Gregory Brower, a former top FBI official. "Mueller and Barr will have to explain this."

Democrats seized on this discrepancy, with House Judiciary Committee Chairman Rep. Jerry Nadler, D-N.Y., tweeting that he would call Barr to testify about his decision.

But the questions cannot change the legal result. Mueller worked for the Justice Department, and he made the decision, according to Barr's letter, to defer to his superiors — like the Marine officer he once was.

Barr, Trump's appointee, suggested the call was not a difficult one.

For one thing, he said in the letter, there was no evidence of the underlying crime of conspiracy, and "the absence of such evidence bears upon the president's intent with respect to obstruction."

He said Mueller didn't identify actions that in Barr's judgment were done with corrupt intent. That corrupt intent, Barr said, is required "under the Department’s principles of federal prosecution guiding charging decisions, would need to be proven beyond a reasonable doubt to establish an obstruction-of-justice offense."

Of course, one way to investigate a defendant's intent is to interview him. Mueller did not interview Trump, and the reason that never happened was not covered in Barr's letter.

The discrepancy on the obstruction question has given Democrats plenty of fodder to question Barr's decisions.

One issue that is bound to come up is the fact that Rosenstein was considered by some a witness on obstruction, because he wrote a memo that Trump cited in firing FBI Director James Comey — a key point in the obstruction case.

But no amount of questioning will change the fact that the Mueller investigation is over. And, as a legal matter, the president is in the clear.