The Justice Department appointed former FBI Director Robert Mueller on Wednesday as a special counsel to oversee a federal investigation into potential coordination between Russia and Donald Trump’s campaign during the 2016 presidential election.

The Justice Department can appoint a special counsel, often called a special prosecutor, any time the person who would ordinarily handle a case is believed to face a conflict or might be perceived to hold potential bias.

Attorney General Jeff Sessions recused himself from involvement in probing the Russia matter, so it was up to Deputy Attorney General Rod Rosenstein to decide whether to appoint a special counsel.

Congress does not have the power to appoint a special counsel, though some Democrats were publicly urging Rosenstein to do so.

Once appointed, a special counsel works with the FBI and the court system to subpoena documents, conduct interviews and potentially seek criminal charges.

Formal Justice Department rules govern how special counsels operate, to provide more independence. Nevertheless, any president willing to face the political consequences still has the authority to dismiss a special counsel.

At one time, officials could appoint what was called an independent counsel, or independent prosecutor, but the post-Watergate-era law behind it was allowed to expire in 1999 amid displeasure over independent investigations of President Bill Clinton.

Under the 1978 statute, the attorney general started the process while the appointment of an independent counsel was formally made by a panel of federal judges from the U.S. Court of Appeals for the District of Columbia Circuit. The judges would then oversee the independent counsel’s work. Under the statute, the independent counsel could be dismissed only for “good cause.”