Special counsel Robert Mueller sent the highly anticipated report summarizing his investigation to Attorney General William Barr on Friday, but it's unclear how much of it will ever be seen by the public or by Congress.

During his confirmation hearing, Barr said he intended to make as much information available as he could, consistent with the special counsel regulations. But he also said Mueller's report itself might not be released. "The special counsel report is confidential, and the report that goes public would be a report by the attorney general," Barr told the Senate Judiciary Committee.

While it's widely assumed that Mueller's report will be a tell-all and that Congress and the public will see most of it, Justice Department regulations actually provide for a very limited report by Mueller — and for even less information to go to Congress or to be made public.

"At the conclusion of the Special Counsel's work," the regulations say, "he or she shall provide the Attorney General with a confidential report explaining the prosecution or declination decisions reached by the Special Counsel," meaning an explanation of the cases being brought to court and of those not being prosecuted.

The rules governing Mueller's work were drawn up in 1999 after the lapse of the Independent Counsel Act, a post-Watergate law that allowed a three-judge panel to appoint a prosecutor who did not act under the supervision of the Justice Department. Kenneth Starr was the best-known independent counsel: His findings led to the impeachment proceedings against President Bill Clinton.

But the law was widely considered a failure, allowing for extended fishing expeditions, and Congress chose to abandon it when the Independent Counsel Act came up for renewal. It had required the submission of a public report when an investigation was completed, and the Justice Department concluded that such a provision should not be carried over in creating the new special counsel regulations.

A Federal Register note explaining the rules said the public report requirement "both 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."

By contrast, the DOJ said, a special counsel would be required only to submit "a summary final report to the Attorney General. This report will be handled as a confidential document, as are internal documents relating to any federal criminal investigation."

Under the old law, an independent counsel was required to report to the House "any substantial and credible information ... that may constitute grounds for an impeachment." No such mandate is in the special counsel rules that govern Mueller's investigation.

As for the report the attorney general sends to Congress, that is intended to be even more limited under the rules. The chairman and ranking minority members of the House and Senate Judiciary Committees must be told when the special counsel's investigation is concluded. That notification must include "a description and explanation of instances (if any) in which the Attorney General concluded that a proposed action by a Special Counsel was so inappropriate or unwarranted under established Departmental practices that it should not be pursued."

In other words, all the rules call for in notifying Congress is an explanation of any action the attorney general took to block something a special counsel wanted to do.

Congressional Democrats and some Republicans have said they will not be satisfied with simply receiving Barr's summary of Mueller's report. They are demanding to see the full Mueller report and say they are prepared to seek a subpoena if he does not turn it over. And a public clamor to see the report may lead Barr to find some way to depart from the rules.

A letter from the leaders of six House Democratic committees said it's especially important to inform Congress if Mueller turned up any evidence that the president committed a crime, given that long-standing Justice Department policy says a president cannot be indicted. "Congress could be the only institution currently situated to act on evidence of the president's misconduct," their letter said.

House investigators have also said they will seek material Mueller presented to a grand jury during the course of his work. Congress is generally not entitled to receive material gathered by a grand jury, which is secret.

One notable exception came in 1974, when a federal judge ruled that a report drawn up by the Watergate grand jury could be sent to a House committee examining President Richard Nixon's possible impeachment. That precedent may have limited value, however. Nixon chose not to object to the grand jury's action. President Donald Trump might not be so accommodating.