Here's the most important question we need answered at the end of Mueller's investigation

Special Counsel Robert Mueller is done investigating, but we still have questions.

Many of those questions regard conduct that is known publicly but hasn't been featured in any of Mueller's indictments. What went on with Erik Prince at the Seychelles meeting? What more is there to know about the Trump Tower meeting with Natalia Veselnitskaya? What is the deal with the so-called "Ukrainian peace plan"? Why did Paul Manafort give a man believed to have ties to Russian intelligence polling data in the middle of the campaign? What, if anything, did President Donald Trump know about these events? I could go on.

But the one most important question that will hopefully be answered soon, subsuming all of the other unresolved threads in the investigation, is this: Why has Mueller ended his probe now?

As Lawfare's Ben Wittes argued, there are many different reasons the investigation could be over, and different answers could have radically different implications

"When a high-profile criminal investigation ends, a certain set of understandings normally guides what conclusions observers can and cannot responsibly draw about those who have escaped uncharged," Wittes explained. "These understandings work because they flow from a common set of assumptions about why the investigation has ended."

One possible answer is that Mueller has determined that there is no evidence that anyone who has not been charged broke the law in ways relevant to his investigation. This appears to be the conclusion many of Trump's defenders are drawing, and it's certainly possible — but it's only one of several explanations, so any celebration is premature. (And, of course, it leaves open the possibility of criminal activity that is being investigated by other parts of the DOJ.)

Wittes noted that "the 'clearing' of the subject is not what the end of a criminal probe normally means."

We know, for example, that Mueller is operating under a Justice Department that holds a sitting president cannot be indicted. So as far as Trump's conduct goes, there's almost nothing we can deduce about the absence of an indictment.

Mueller's report will address his prosecutions and his declinations to prosecute. In other words, he'll talk about the people and conduct he considered indicting but refrained from doing so. We already know most of what we're going to know about the prosecutions, though there could be more details forthcoming — perhaps Mueller's suspicions about other crimes the defendants committed, even if there wasn't sufficient evidence to bring charges — so it's his declination decisions that will likely be most interesting. Wittes explained:

As a general matter, declinations fall into three broad categories: factual declinations, legal declinations, and prudential declinations. These overlap to some degree, and the distinctions here are thus somewhat artificial—though I hope still useful. A declination for factual reasons could be based on a finding of actual innocence—as discussed above—or a finding that the evidence, though compelling, is not adequate for prosecution. How much of a vindication or how politically damaging such a declination is depends entirely on the factual findings, about which we know nothing

One interesting possibility is "prudential declinations." Wittes pointed out that protecting intelligence sources — a factor certainly implicated in this case — can motivate prosecutors to refrain from charging otherwise serious crimes because the source is more important than a conviction. It seems unlikely this would be could apply in the worst-case scenario — if Trump were an outright Russian asset, any intelligence source would be worth burning to expose that fact. But it could explain why, for instance, Manafort isn't charged with an election-related conspiracy, when Mueller was able to bring charges on other matters.

Wittes also notably argued that even in the best-case scenario for Trump and his allies — that the declination of further prosecutions means that the facts don't support any further charges — they may still be open to moral condemnation for the actions uncovered in the investigation.

Ultimately, though, the country has a deep interest in why Mueller has wrapped up. The report will provide that answer, which is why it should be released.