On Friday, when announcing the appointment of Jack Smith as special counsel overseeing investigations of Donald Trump’s alleged election subversion and retention of White House records, the US attorney general, Merrick Garland, said the selection would ensure “independence and accountability in particularly sensitive matters”.

So why did Garland take this step against the former president?

What is a special counsel?

Special counsels are usually highly experienced federal prosecutors. According to justice department regulations, a special counsel is appointed when an attorney general “determines that criminal investigation of a person or matter is warranted” but “investigation or prosecution of that person or matter by a United States attorney’s office or litigating division of the [justice department] would present a conflict of interest … or other extraordinary circumstances”.

An attorney general must therefore determine that it is “in the public interest to appoint an outside special counsel”.

Have those tests been met?

Garland says they have.

Trump’s attempts to overturn his defeat by Joe Biden in 2020, including inciting the Capitol attack on 6 January 2021, have been exhaustively documented. His retention of White House records, many classified, has been established through an FBI search of his Mar-a-Lago resort, among other incidents.

But such matters are certainly politically sensitive. Citing “recent developments” including Trump’s announcement that he is running for president again and Biden’s “stated intention to be a candidate as well”, Garland said he had “concluded that it is in the public interest to appoint a special counsel”.

This, Garland said, would “underscore the department’s commitment to both independence and accountability in particularly sensitive matters. It also allows prosecutors and agents to continue their work expeditiously, and to make decisions indisputably guided only by the facts and the law”.

How do special counsels work?

Outlining how Smith will work “quickly and completely”, Garland quoted from department regulations: “Although the special counsel will not be subject to the day-to-day supervision of any official of the department, he must comply with the regulations, procedures and policies of the department.”

Are special counsels completely independent?

No. Regulations also state that the attorney general can request explanation of any step taken and direct it not be pursued. If that happens, the attorney general must notify Congress. Special counsels and their staff are also subject to department disciplinary procedures.

Who can fire a special counsel?

Regulations say a special counsel “may be disciplined or removed from office only by the personal action of the attorney general”. He or she can do this “for misconduct, dereliction of duty, incapacity, conflict of interest or for other good cause, including violation of departmental policies. The attorney general shall inform the special counsel in writing of the specific reason”.

Wasn’t Robert Mueller a special counsel?

He was. Appointed in May 2017, the former FBI director investigated “Russian government efforts to influence the 2016 presidential election and related matters”, including links between Trump and Moscow.

Didn’t Trump try to fire him?

Trump did. But only the attorney general can do so, so it didn’t work. Attempts to get rid of Mueller featured among examples of potential obstruction of justice which Mueller laid out.

Wasn’t there another special counsel?

Yes. Trump’s second attorney general, William Barr, appointed John Durham to investigate justice department activities which gave rise to the Russia investigation. Durham’s work now appears to be winding down, without having produced major indictments. The two cases he took to trial ended in acquittals.

What happens when a special counsel is done?

The attorney general decides how to proceed. In Mueller’s case, critics charge, Barr misrepresented the special counsel’s findings in order to let Trump off the hook. Whether he wriggles off it this time will be up to Garland.