WASHINGTON — The swirl of speculation surrounding the Russia investigation often assumes that the special counsel, Robert S. Mueller III, will release a report of his findings that will serve as the definitive explanation of how Russia interfered in the 2016 election and whether President Trump or his associates coordinated with Moscow.

But there is no such guarantee. The law does not require the Justice Department to release a report, and Mr. Mueller has been silent on the issue. Mr. Trump’s nominee for attorney general, William P. Barr, said at his confirmation hearing on Tuesday that he wanted to release as much of what Mr. Mueller found as possible. But he said he needed to learn more about the report and the regulations that govern his releasing information from it before deciding what to do about disclosing the findings.

That answer did not satisfy leading Senate Democrats, who said on Wednesday that they would oppose Mr. Barr’s nomination unless he agreed to release the entire report Mr. Mueller produces, except for redactions of sensitive national security information.

Why do people assume that a Mueller report is coming?

Because the government has issued plenty of big reports after important investigations into national catastrophes and scandals. Commissions that investigated the Sept. 11 attacks and the failure to find weapons of mass destruction after the invasion of Iraq, for example, produced book-length public reports that became part of the historical record.