John Q. Barrett is a law professor at St. John’s University in New York City. He was associate counsel in the Office of Iran-contra Independent Counsel Lawrence E. Walsh and a Justice Department attorney.

Wednesday’s sentencing of Michael Cohen to three years in prison appears to mark the end of the Justice Department’s interest in investigating and prosecuting the president’s former lawyer and “fixer.” But Cohen’s time in the public eye is not over.

In connection with his guilty pleas, Cohen provided some information to the U.S. Attorney’s Office for the Southern District of New York about his tax evasion and campaign finance crimes. He also gave information to the office of special counsel Robert S. Mueller III — somewhat more expansively and usefully — about his lies to Congress, President Trump, his possible crimes, Trump business activities, contacts with Russians during the Trump presidential campaign, and other topics.

What Cohen told those Justice Department offices is known, fully, only to them. But Congress, and especially the incoming Democratic House majority, can seek and obtain that information from Cohen.

And now that the criminal justice system is done with him, Congress can gather that information — plus the testimony of others whom Justice is done with — without impairing criminal law enforcement.

This sequencing — the Justice Department going first and then Congress investigating — offers a unique opportunity for oversight, accountability and public information about criminal conduct close to the White House and potentially involving the president.

Congress has great latitude to hold oversight hearings. These include powers to subpoena witnesses to testify and to produce tangible evidence. When witnesses respond to Congress by claiming Fifth Amendment rights not to incriminate themselves, federal law gives Congress the power to get court orders granting immunity to such witnesses and compelling them to cooperate.

In the modern era, past evidence of possible crimes by presidents and their close associates provoked concurrent congressional oversight and executive branch criminal law-enforcement investigations.

Take Watergate: In 1972 and early 1973, the Justice Department prosecuted people who were involved in breaking into Democratic National Committee offices. When it became clearer that much more was involved, the Senate commenced its Watergate investigation — the Ervin Committee. This occurred alongside Attorney General Elliot Richardson appointing Archibald Cox to be Watergate special prosecutor, to take over and expand the Justice Department’s criminal investigations as facts justified.

The same was true of Iran-contra. Beginning when U.S. military support to Nicaraguan contras and then U.S. arms sales to Iran were first revealed in late 1986, Justice Department components began to investigate. When DOJ discovered possible White House crimes, it sought the appointment of an independent counsel (the gentler name for a special prosecutor, under the law then in effect). Simultaneously, the Senate and House intelligence committees began investigating, and in 1987 Congress created the Iran-contra select committees to take over that role.

In each instance, the congressional inquiries sometimes helped and often hindered the course of law enforcement. In Watergate, Senate staff uncovered President Richard M. Nixon’s White House taping system — that was extremely helpful to both Congress and the special prosecutors (first Cox, then his successor Leon Jaworski), and in the end it was fatal to Nixon’s presidency.

But along the way, the Senate compelled immunized testimony that made some criminal prosecutions (e.g., John Dean’s) difficult and others futile. The Senate also obtained voluntary public testimony from others (e.g., John Ehrlichman, H.R. Haldeman and John Mitchell) that prosecutors had to navigate in later trials.

The same occurred during Iran-contra. Over protest from Independent Counsel Lawrence E. Walsh (for whom I worked), the select committees compelled people who had invoked Fifth Amendment rights (principally Oliver North and John Poindexter) to testify with immunity. Although each was later prosecuted criminally and convicted, appellate courts held that the possibility that immunized testimony affected witness recollections made their convictions invalid. The select committees also elicited considerable testimony about Iran-contra that turned out to be wrong, complicating and prolonging Walsh’s work as prosecutor.

The incoming Congress will have a unique opportunity to figure out a lot about what, if anything, Trump and others did with Russians and others. And this time Congress can do so without impairing federal criminal law enforcement.

Congress should start by calling low-level witnesses — people who face no threat of prosecution — to testify, laying factual groundwork and raising the heat on others who have relevant information.

Then Congress can, with little or no cost to law enforcement, call more significant players to testify. In doing this, Congress should seek any guidance, on witnesses and timing, that Mueller will provide. Then Congress should question these witnesses in public for as long as it takes to get their relevant information in full.

Cohen might be in custody soon, but he can be brought from his cell to Capitol Hill. And Congress can, if necessary, get a court order compelling him to testify.

Michael Flynn may soon be in the same position. He will be sentenced next week for his crime of lying to the FBI about his contacts with Russian officials. Assuming he tells the truth, he will not be prosecuted again. He will be available to inform Congress and the public about what he knows.

Others who have pleaded guilty to charges already or might do so soon are in similar positions — available to congressional oversight without threat to Mueller’s investigation.

It is never ideal for congressional investigation of government corruption to hang back, waiting for federal law enforcement.

But when Congress has done that, and when the criminal justice process then finishes up with central players who have probative information, such people are being served up for Congress to perform its government oversight role without thwarting criminal law enforcement.

In the Trump matters, for the national interest, Congress should get to it, with energy and skill.

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