WASHINGTON — Millions of Americans are waiting for Robert Mueller to give them the final word on whether the Trump campaign conspired with the 2016 Russian election interference effort — and whether their president is under the influence of a foreign adversary.

Millions of Americans may be sorely disappointed.

Unless Mueller files a detailed indictment charging members of the Trump campaign with conspiring with Russia, the public may never learn the full scope of what Mueller and his team has found — including potentially scandalous behavior that doesn't amount to a provable crime.

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The reason: The special counsel operates under rules that severely constrain how much information can be made public.

Those rules require that the special counsel's report to the attorney general be "confidential." And, while the attorney general is required to notify Congress about Mueller's findings, the rules say those reports must amount to "brief notifications, with an outline of the actions and the reasons for them."

"Expectations that we will see a comprehensive report from the special counsel are high. But the written regulations that govern the special counsel's reporting requirements should arguably dampen those expectations," said Chuck Rosenberg, a former federal prosecutor and NBC News analyst.

When Independent Counsel Kenneth Starr released the report of his investigation of President Bill Clinton in 1998, all of Washington paused to digest the 453-page document (plus 2,000 pages of appendixes), with its salacious details of the president's sexual dalliance with an intern. It was made public at the same time it was sent to Congress.

The Mueller report won't be anything like that. Starr operated under the now-defunct independent counsel law, meaning he called many of his own shots, outside the purview of the Justice Department. Mueller is a special counsel under Justice Department supervision, subject to very specific regulations.

Here is the sum total of what the rules say about a final report:

"At the conclusion of the Special Counsel's work, he or she shall provide the Attorney General with a confidential report explaining the prosecution or declination decisions reached by the Special Counsel."

What's more, a 1999 document outlining that rule in the federal register criticizes the way in which independent counsel reports like Starr's were made public, saying that a public prosecutors report "provides an incentive to over-investigate, in order to avoid potential public criticism for not having turned over every stone, and creates potential harm to individual privacy interests."

Memos explaining decisions not to prosecute can be long or short, and there is nothing in the rules to prevent Mueller from writing a 500-page narrative laying out the behavior of the Trump team with regard to Russia in excruciating detail.

However, that report will go to the attorney general, and under the regulations, it is secret.

The attorney general is required to send a report to Congress. But here is what the rule says about that:

"To help ensure congressional and public confidence in the integrity of the process, the regulations impose on the Attorney General these reporting requirements to the Judiciary Committees of the Congress. These reports will occur on three occasions: on the appointment of a Special Counsel, on the Attorney General's decision to remove a Special Counsel, and on the completion of the Special Counsel's work.

"These reports will be brief notifications, with an outline of the actions and the reasons for them."

There is a wildcard — if the Mueller report contains allegations of potentially impeachable offenses against the president, scholars have said the Justice Department would have to pass the full details of that to Congress.

But short of that, it's not clear Congress will get access to the evidence Mueller has gathered.

Mueller has filed charges against 34 people and secured convictions of some of Trump's key former advisers. But none of the charges have accused anyone in Trumpworld of conspiring with the Russian intelligence operation to help Trump get elected in 2016. Nor has Mueller made any allegation that the president is compromised by Russia — a suspicion that percolates in part because of Trump's favorable behavior towards Vladimir Putin.

What if Mueller has evidence of that, but not enough to prove a criminal case? Isn't the public entitled to know?

The Democrats who now control the House have already made clear that they will do everything they can to ensure that Mueller's findings become public. They may well subpoena the confidential Mueller report to the attorney general.

The Senate Intelligence Committee, which is conducting its own investigation of the 2016 election, would also have an interest in gaining access to Mueller's findings and the evidence he has obtained.

But the Justice Department may well resist those demands. Congress generally can't get access to information gathered by a grand jury, which by law is secret. And that may cover large swaths of the information Mueller has gathered.

Other aspects of the report are likely to be classified or "law enforcement sensitive," a term of art for confidential information.

Still other segments may be covered by executive privilege.

William Barr arrives for a meeting with Sen. Josh Hawley on Jan. 29, 2019 in Washington. Mark Wilson / Getty Images file

What's more, a central principle of the U.S. justice system is that prosecutors don't pass public judgment on conduct unless they are doing so in a criminal filing. Former FBI Director James Comey was criticized for doing that in the Hillary Clinton email investigation.

Multiple Justice Department and Congressional officials have told NBC News they expect a Mueller report to be sent over to the Justice Department in the next several weeks.

When it lands, it's possible officials will publicly acknowledge receipt.

And that will be it, until Attorney General William Barr decides how, if at all, he is going to make information public.

Barr was asked about this many times at his confirmation hearing. And, as legal scholars Jack Goldsmith and Maddie McMahon pointed out in an article on Lawfare.com, Barr promised to make as much information public as he could. But he added an important qualifier: "consistent with current law."

"Barr was remarkably — and, in our view, understandably — noncommittal in these statements," they write. "He reminded the senators of what the regulations say and noted that he would follow them.

Barring the distinct possibility of a leak, Barr may play a crucial role in deciding how much of Mueller's findings become public. The rules governing the special counsel are internal rules that Barr could change. And they do not appear to constrain the attorney general from holding a news conference and discussing Mueller's report.

But nothing Barr said during his confirmation hearing suggested he would do that.

"Let's assume that Mr. Mueller at some point, hopefully soon, writes a report and that report will be given to you," Sen. John Kennedy asked him. "What happens next under the protocol rules and regulations at Justice?"

Barr replied: "Well under the current rules, that report is supposed to be confidential and treated as, you know, the prosecution and declination documents in an ordinary—any other criminal case. And then, the attorney general as I understand the rules, would report to Congress about the conclusion of the investigation. I believe there may be discretion there about what the attorney general can put in that report."

How Barr exercises that discretion may determine how much the public learns about what Robert Mueller knows.