The Washington Post's Devlin Barrett explains the Justice Department's decision to appoint Robert Mueller as special counsel to investigate possible connections between the Trump campaign and Russian officials. (Peter Stevenson,Jason Aldag,Whitney Leaming/The Washington Post)

The Justice Department appointed former FBI director Robert S. Mueller III on Wednesday to serve as a special counsel at the head of an investigation into possible collusion between the Trump campaign and Russia.

The move figures to appease the dozens of Senate Democrats who called for an independent probe after President Trump fired James B. Comey as FBI director last week. (Mueller was Comey's predecessor.) With Trump wielding the power to nominate the next FBI director, who would then complete the investigation of Trump's campaign, the appearance of a conflict of interest seemed unavoidable.

Thus, the idea of a special prosecutor makes sense, in theory. In practice, some investigations headed by special prosecutors have rung up huge tabs while producing modest results.

Josh Chafetz, a professor at Cornell Law School, offers this reminder: “The special prosecutor's agreement with the attorney general may specify certain types of independence but, at the end of the day, the prosecutor works for the Department of Justice and can be fired by the attorney general or the president.”

Such firings have actually happened, by the way. Here's a brief history of special prosecutors:

Whiskey Ring Scandal

In 1875, Treasury Secretary Benjamin H. Bristow exposed a conspiracy by a group of whiskey distillers in St. Louis, Chicago and Milwaukee to evade federal taxes. The distillers had bribed IRS officials and planned to use unpaid tax money to help fund the reelection campaign of President Ulysses S. Grant.

Grant, who was not suspected of involvement, appointed special prosecutor John B. Henderson, whose investigation led to 238 indictments — including of Grant's personal secretary, Orville E. Babcock. Grant fired Henderson and replaced him with James Broadhead. Grant testified in Babcock's defense, and Babcock was acquitted; 110 others were convicted on charges related to tax fraud.

Star Route Scandal

President James Garfield appointed special prosecutor William Cook in 1881 to investigate the awarding of postal contracts for so-called star routes in the West and Southwest. Cook's probe produced 25 indictments on charges related to bribery but yielded only convictions, both of which were reversed in a retrial.

More Post Office bribery

In 1903, President Theodore Roosevelt's attorney general, Philander Knox, appointed two special prosecutors, Charles J. Bonaparte and Holmes Conrad, to investigate another bribery scheme at the Post Office. Thirty Post Office officials and private contractors were indicted, but the postmaster general, Henry C. Payne, survived the scandal.

Oregon land fraud

Knox appointed another special prosecutor, Francis J. Heney, in 1905 to investigate a land fraud scheme in Oregon. Almost 100 people were indicted, including Sen. John H. Mitchell (R-Ore.) and Rep. John N. Williamson (R-Ore.), both of whom were convicted. Williamson won his appeal; Mitchell also appealed but died before the case concluded.

Teapot Dome Scandal

In 1924, President Calvin Coolidge appointed special prosecutors Atlee Pomerene and Owen Roberts to probe the leasing of oil fields, including the Teapot Dome reserve in Wyoming. Interior Secretary Albert Bacon Fall was convicted of bribery five years later, but served just nine months in prison.

Justice Department corruption

After a congressional probe uncovered a conspiracy among officials at the Justice Department and Bureau of Internal Revenue to fix tax cases, President Harry S. Truman appointed special prosecutor Newbold Morris in 1952. Morris was given the title special assistant attorney general.

As part of his investigation, Morris demanded that roughly 600 Justice Department officials fill out detailed personal finance questionnaires. Attorney General J. Howard McGrath refused and fired Morris, who had been on the job for only two months. McGrath then resigned, and Truman did not name another special prosecutor.

In the dark days of the Watergate scandal, President Richard Nixon pushed out two attorney generals and the special prosecutor of the Watergate investigation in what became known as the "Saturday Night Massacre." (Jenny Starrs/The Washington Post)

Watergate

In 1973, Attorney General Elliott Richardson appointed special prosecutor Archibald Cox to lead the Watergate investigation. Cox subpoenaed Oval Office audio recordings made by President Richard M. Nixon, but Nixon fought the subpoena in court. After a federal appeals court ordered Nixon to turn over the tapes, he fired Cox. Richardson and Deputy Attorney General William D. Ruckelshaus resigned in protest, in what became known as the Saturday Night Massacre.

Nixon initially said his Justice Department would take over the Watergate investigation but, under pressure, he appointed another special prosecutor, Leon Jaworski, who ultimately succeeded in obtaining the tapes via a Supreme Court ruling. Nixon resigned in August 1974.

Iran-contra affair

In 1986, a three-judge panel appointed independent counsel Lawrence E. Walsh to investigate the Ronald Reagan administration's sale of weapons to Iran. (The Ethics in Government Act of 1978 had replaced the term “special prosecutor” with “independent counsel.” Attorneys general retained the power to decide when a special prosecutor was necessary, but judiciary panels assumed selection authority.)

Walsh concluded that the Reagan administration had illegally sold the weapons to secure the release of U.S. hostages and defied Congress by giving the proceeds to Nicaraguan rebels, known as contras, who were fighting to overthrow their Marxist government. Fourteen U.S. officials faced criminal charges and 11 were convicted, including Reagan's national security adviser, Navy Adm. John M. Poindexter, and Marine Lt. Col. Oliver North, the National Security Council staff aide accused of masterminding the scheme.

The convictions were overturned on appeal, however, and other officials received presidential pardons. A $39 million investigation ultimately resulted in zero prison time.

Whitewater

In 1994, Attorney General Janet Reno appointed Robert B. Fiske Jr. special counsel (yes, another new term) to investigate the business dealings of the Whitewater Development Corp., formed by Bill and Hillary Clinton and two partners after their joint purchase of 220 acres of land in Arkansas’ Ozark Mountains in 1978. The Ethics in Government Act had lapsed by the time Reno tapped Fiske; when it was renewed later in 1994, a three-judge panel replaced Fiske with Kenneth W. Starr.

Starr's investigation led to fraud convictions for the Clintons' partners, James B. and Susan McDougal. The Clintons were never charged. But Starr's probe expanded to include the president's personal conduct, ultimately focusing on his affair with White House intern Monica Lewinsky and his statements under oath about the relationship. The House of Representatives impeached Bill Clinton in 1998, accusing him of perjury and obstruction of justice, but the Senate acquitted him in 1999.

The Whitewater investigation cost $70 million.

Valerie Plame

In 2003, Attorney General John D. Ashcroft recused himself from an investigation into whether George W. Bush administration officials illegally leaked the name of CIA operative Valerie Plame to a journalist. The Ethics in Government Act had lapsed again, leaving the Justice Department with the authority to name a special prosecutor.

With Ashcroft on the sideline, the decision fell to his 43-year-old deputy, who had been on the job for just three weeks — James B. Comey.

Comey appointed Patrick J. Fitzgerald, who sought the cooperation of New York Times reporter Judith Miller in identifying the source of the leak. Miller refused because she had promised to protect a confidential source and in 2005 spent almost three months in jail for contempt of court. Miller's source, I. Lewis “Scooter” Libby, chief of staff to Vice President Richard B. Cheney, ultimately released her from their confidentiality agreement and was convicted in 2007 on charges related to the leak.

Amber Phillips contributed to this report. This post has been updated.