Special Counsel Robert Mueller's investigation of Russian meddling in the 2016 presidential election and possible collusion with Donald Trump's campaign has just entered its second year. It has resulted in criminal indictments of several individuals and Russian companies and a number of other actions.

The President wants the investigation to end. But history suggests that it needs to continue for as long as it takes to expose what really happened and outline actions to prevent recurrence.

In October 2016, before the presidential election, several U.S. intelligence agencies reported that the Russians had been meddling in the election process. After the election, president Obama expelled a number of Russian diplomats in retaliation. The House Intelligence Committee and the Senate Intelligence Committee have both investigated and issued reports confirming Russian interference, but differing on whether the Russians were trying to help Trump. Additional Senate reports are expected in the future. But judging from what has been released so far, neither the House report nor Senate reports seem likely to be definitive or fully revealing about what happened and why.

Getting to the bottom of things takes time, as the following examples indicate.

Pearl Harbor – 4 1/2 years

The Japanese bombed the American naval and air bases at Pearl Harbor, Hawaii without warning on December 7, 1941, a surprise attack that brought the United States into World War II, against Japan and its ally Germany. The U.S. government conducted eight official inquiries into the attack between 1941 and 1945, including one by the Army, one by the Navy, one by the Secretary of War, and one under the direction of President Franklin D. Roosevelt.

As a result of the probes, the Army and Navy commanders at Pearl Harbor were removed for dereliction of duty. Other actions were taken to tighten military intelligence and security. The war was fought to a victorious close by September 1945.

But the public was not satisfied with the official explanation. Why exactly had America been caught by surprise? Congress decided more was study was needed. It launched its own investigation, by a joint Senate-House committee, from November 1945 through May 1946. The congressional study included testimony from 44 witnesses, which filled more than 5,000 pages of transcripts, and reprinted many documents, some of which had not been available to previous investigations.

The report was issued in June 1946, four and a half years after the attack. The report, signed by eight of the ten joint committee members, concluded that "the ultimate responsibility for the attack and its results rests upon Japan,” and “the diplomatic policies and actions of the United States provided no justifiable provocation whatever for the attack by Japan on this Nation.” It noted that "officers, both in Washington and Hawaii, were fully conscious of the danger from air attack” but not Japan's specific plans. The army and navy commands in Hawaii and the War and Navy Departments made “errors of judgment and not derelictions of duty.” The authors rejected the claim that President Roosevelt and top advisors “tricked, provoked, incited, cajoled, or coerced Japan” into attacking the United States in order to draw the nation into war.

Two members of the committee issued a dissent, laying more of the responsibility on Roosevelt and other Administration leaders. But the public reaction to the majority report was positive. The report was thorough, well documented, and persuasive.

JFK Assassination – 54+ years

The assassination of president John F. Kennedy in Dallas, Texas, on November 22, 1963, was investigated by the Dallas police and the FBI. President Lyndon Johnson appointed a special commission, headed by Chief Justice Earl Warren, which reported in September 1964, ten months after the murder, that Lee Harvey Oswald was the lone assassin. Critics charged the Commission had moved too fast and its findings were immediately challenged.

In 1968, Attorney General Ramsay Clark conducted another investigation of the shooting. President Gerald Ford had Vice President Nelson Rockefeller conduct another probe in 1975 as part of a broader investigation into CIA activities. A Senate investigation headed by Senator Frank Church looked into the assassination again that same year. The House of Representatives in 1976 established a committee to probe the assassinations of president Kennedy and Martin Luther King, which issued a report three years later.

Each of the investigations reviewed and interpreted available evidence differently and resulted in a somewhat different conclusion.

In 1992, Congress established a board to oversee the collection of all official government records relating to the assassination. The board completed its work in 1998. All remaining records were to be opened by the National Archives in 2017. Many were released last summer and fall, but many of them included redactions. But even then, 520 files were kept sealed because of possible national security concerns.

The Warren Commission was probably right. But the multiple investigations, inconsistencies, contradictions, allegations of a possible conspiracy, and controversies over release of records, stretching over more than fifty years, have left the impression that the full story has never been revealed.

Watergate – 2 years

A break-in at the Democratic Party office at the Watergate complex in Washington occurred on June 17, 1972. Washington D.C. police and FBI investigations soon revealed that the intruders had connections to the committee to re-elect President Richard Nixon. Nixon administration officials and the president himself began taking action to cover up their involvement. The Senate appointed a special committee to investigate and hold hearings. An administration official testifying before the committee revealed the existence of a secret tape recording system in the White House. By then, the Attorney General had appointed a Special Prosecutor to investigate. When the prosecutor demanded the tapes, Nixon refused and ordered him fired. The Attorney General resigned in protest; the Deputy AG also resigned rather than take the action; and the third-ranking official in the Justice Department complied. But he appointed a replacement after a public outcry about the dismissal, and the new Special Prosecutor also demanded the tapes. Nixon refused, but the Supreme Court ruled that he had to release them. In the meantime, a grand jury in Washington had indicted several Nixon aides for their role in obstructing the investigation. One of the tapes revealed that Nixon himself had helped orchestrate the cover-up. The House of Representatives began impeachment proceedings.

Nixon resigned on August 8, 1974.

That was more than two years after the break-in, and it had required sustained action by the Justice Department, both houses of Congress, and the courts.

But the efforts had brought the facts to light, penetrated the cover-up attempts by Nixon and his staff, and exposed criminal activity. Nixon was later pardoned by his successor, Vice President Gerald Ford, but several of his aides served prison terms. The Watergate episode tarnished the presidency, but the multiple investigations and legal actions left the public satisfied that the facts had come out into the open.

9/11 — 3 years

After the terrorist attacks on 9/11/01, the House and Senate launched a joint committee to investigate the causes of the attacks. The committee issued a lengthy report in December 2002. Despite its length, it was superficial and inconclusive on what caused the attacks and how the terrorists had carried them out. Families of victims and others demanded more and this pressure led president Bush and Congress to create the National Commission on Terrorist Attacks Upon the United States, an independent bipartisan commission, popularly known as the "9/11 Commission." The Commission carried out a thorough investigation and issued its comprehensive, in-depth report in July, 2004.

But that was nearly three years after the attacks.

The Pearl Harbor, JFK, Watergate, and 9/11 investigations suggest that:

â— Thorough, detailed, objective, well-documented investigations are better than what may seem like quick rush-to-judgment probes. A longer and more thorough initial investigation of the JFK assassination than that carried out by the Warren Commission might have provided a more definitive explanation of the tragedy and made later investigations unnecessary. The 9/11 Commission produced something much more probing and substantive than the House/Senate report issued a year after the attack. Likewise, Mueller's report, when finished, can be expected to dig deeper and reveal more than the House and Senate reports.

â— The media are an important supplement to official investigations. The Washington Post's and other newspapers'Watergate reporting helped sustain public interest, encourage congressional action, and help persuade Nixon to leave. The 1991 movie "JFK" rekindled media and public interest in the assassination and helped encourage Congress to create the assassination records review board. Media coverage of Russian meddling in 2016 has helped sustain Mueller.

â— There may be precedents in these four investigations for Mueller's probe, e.g., which official documents can be released to the public, and when ( a central issue in the JFK reports); the role and authority of grand juries (for instance, one Watergate grand jury named president Nixon as an "unindicted co-conspirator"); and the implications of perjury and obstruction of justice (major issues in Watergate).

â— Explaining the causes of past events helps deepen public insights (think 9/11), bring closure (think Pearl Harbor) and sometimes produces dramatic results (think Watergate, which ended with the resignation of the president).

â— But what may count most is the usefulness of the reports in preparing for the future. For instance, the recommendations of the Congressional Pearl Harbor report provided the basis for the National Security Act of 1947. The law consolidated the nation's military forces into a new Department of Defense and established the Central Intelligence Agency to gather and analyze intelligence related to national security.

Traumatic historical events merit thorough investigation.

Robert Mueller needs to keep working and the public needs to be patient.

Dr. Bruce W. Dearstyne is a historian in Albany, NY. SUNY Press published his book The Spirit of New York: Defining Events in the Empire State's History in 2015.