President Donald Trump in a meeting with business leaders. Photo by Chip Somodevilla/Getty Images After FBI Director James Comey's unexpected firing Tuesday, lawmakers renewed calls for a special prosecutor to investigate ties between Trump associates and Russian operatives.

"The only way the American people can have faith in this investigation is for it to be led by a fearless, independent special prosecutor," Senate Minority Leader Chuck Schumer said.

Democratic senators have called repeatedly for a special prosecutor, more often called an independent or special counsel, to be appointed.

But what exactly is a special prosecutor, how does he or she get appointed, and what happens next? We broke it down.

Who appoints a special prosecutor?

Secretary of State Rex Tillerson (L) and Attorney General Jeff Sessions (R) arrive for US President Donald Trump's first address to a joint session of Congress on the floor of the House of Representatives in Washington, U.S., February 28, 2017. REUTERS/Jim Lo Scalzo/Pool

A special counsel could be appointed by either Attorney General Jeff Sessions himself or by Congress to investigate potential ties between Trump's inner circle and Russia, said William Banks, a professor and the founding director of the Institute for National Security and Counterterrorism at Syracuse University.

A "special counsel" is a modern day term for a "special prosecutor," according to Banks, and any investigation would likely use the term "special counsel." The term "special prosecutor" was used up through the 1980s, after which the laws around special prosecutors expired and were not renewed, therefore retiring the term.

After revelations of previously undisclosed conversations with Russia's ambassador to the US, Sergey Kislyak, Sessions recused himself from investigations relating to the 2016 campaign.

Senate Minority Leader Chuck Schumer Mario Tama/Getty Images

If the attorney general recuses himself, it falls to the deputy attorney general to appoint an independent counsel, according to the Code of Federal Regulations. The appointment of a special counsel by the attorney general or deputy attorney general is "unreviewable," according to the Center for Legal and Economic Studies.

Investigations are currently underway in the Senate and House intelligence committees, but Banks said he believes it is unlikely a special counsel would be created until those investigations conclude.

The other way to establish an independent counsel goes through Congress.

Congress could initiate the process to create a different independent counsel for investigations by passing a law, as it did in 1978, when the Ethics in Government Act was passed. The law dictated that a three-judge panel based at the US Court of Appeals in DC would appoint the counsel. The law, which was reauthorized several times until its sunset in 1999, was used more than a dozen times to initiate investigations, according to PBS Frontline. It was used most famously in the 1990s to appoint attorney Kenneth Starr to oversee investigations in to President Bill Clinton.

Such a law would have to be either signed by Trump or, in the event of a presidential veto, overridden by a two-thirds majority of both houses of Congress. There is precedent, however, for a president to sign an independent counsel law amid scrutiny. Clinton signed a reauthorization of the 1978 law in 1994 with a number of alleged scandals brewing.

Congress could, however, launch its own investigation into the executive branch without legislation because such authority is implicit in the appropriations power, Banks said. If Congress decided to act on its own, it is much more likely that it would establish a commission or committee to investigate, rather than passing ethics legislation, Banks added.

What kinds of people are appointed to a special counsel?

Special counsels tend to be highly respected lawyers or judges. Examples, according to Banks, include: highly experienced private practice lawyers, retired judges, and former Justice Department prosecutors.

How long would a special counsel investigation take to complete?

A special counsel investigation would likely take between six to nine months, according to Banks, who said that such investigations tend to be extremely complicated by nature. With so much classified information, intelligence agency officials that need to be interviewed, and hard to obtain information, it takes a while to sort out.

What does a special counsel have access to?

A special counsel investigation would involve arranging access to classified documents. This could be achieved by either declassifying information or creating clearance to classified documents for the purpose of the investigation only. If the latter is done, it is unlikely the public would see the documents obtained.

A special counsel would also be expected to interview a vast range of people with knowledge of or connection to the investigation.

President Donald Trump speaks on the phone with Australian Prime Minister Malcolm Turnbull in the Oval Office of the White House, January 28, 2017 in Washington, DC. Also pictured at right, National Security Advisor Michael Flynn and White House Chief Strategist Steve Bannon. Drew Angerer/Getty Images

In the case of the Trump-Russia allegations, a special counsel would look into any and all classified or declassified documents that the FBI, CIA, and various police departments and investigation groups might have the incident.

This would include human or digital intelligence, and the dossier delivered by British intelligence. Extensive interviews would be run with anyone close to the situation, including Trump's inner circle, and anyone who had access to digital or technical related information, said Banks. The special counsel themselves would ultimately determine which evidence to use.

What happens after the special counsel investigation concludes?

What happens next depends in part on who appoints a special counsel. Sessions would decide whether the special counsel appointed under him had enough evidence to prosecute Trump or implicated officials.

If Congress created an office for an independent or special counsel, it is likely that the counsel would refer results of the investigation to Congress, though that could change depending on the legislation passed. If Congress initiated an investigation through a commission or committee, it would fall to the attorney general to decide whether to prosecute based on the results provided.

Why are people asking for a special counsel?

Trump and his inner circle have been accused of having close ties to Russia. The White House has denied many of those accusations. Business Insider has previously reported that:

Trump and several associates continue to draw intense scrutiny for alleged ties to and communications with the Russian government.

A dossier of unverified claims alleged serious misconduct in the final months of the 2016 presidential campaign. The White House has dismissed the dossier as fiction, and most of the claims remain unverified.

Trump's campaign aides were accused of having frequent contact with Russia in a report released by the New York Times.

A report published on Wednesday by the Washington Post said that Attorney General Jeff Sessions met with the Russian Ambassador twice during the 2016 election.

Michelle Mark contributed reporting on this article.