Special counsel: What is it and what will Robert Mueller do?
A special counsel has been appointed to oversee the investigation looking into alleged Russian interference in the US presidential election, and if Trump campaign figures were complicit.
The appointment of Robert Mueller, who headed the FBI for more than a decade, came just over a week after President Donald Trump fired FBI director James Comey - sparking calls for such a move.
But what is a special counsel? And what will he do?
An independent overseer
The special counsel was appointed by Rod Rosenstein, the deputy attorney general. He made the decision as "acting attorney general" because Attorney General Jeff Sessions has stepped aside from the Russia inquiry.
Mr Rosenstein said that given the "unique circumstances", it was in the public interest for a special counsel independent "from the normal chain of command", to lead the investigation.
By placing authority for the probe into the hands of Mr Mueller, the idea is that it will be able to proceed without any interference, including from the White House.
OK, but what will he actually look at?
According to Mr Rosenstein's order, he will look into:
- The Russian government's efforts to interfere in the election
- Any links or co-ordination between Russia and Trump campaign-linked individuals
- Any matters that arose or may arise directly from the investigation
That last point gives him quite a broad remit, with observers suggesting that he will also determine if the president himself has committed any wrongdoing.
"This is the way we are going to learn about whether there was an obstruction of justice or whether Mr Trump has violated the law in a way that would require action against him," Professor Stefan Halper, who served as a White House official in the Nixon, Ford and Reagan administrations, told the BBC.
- Mueller 'is ramrod straight in integrity'
- Russia: The scandal Trump can't shake
- How Trump's Russia trouble unfolded
- Could Trump be guilty of obstruction of justice?
What powers does he have?
The special counsel has the powers of a US attorney - meaning he can subpoena records and bring criminal charges.
He can also prosecute anyone who interferes in his investigation through crimes including perjury, obstruction of justice, destruction of evidence, and intimidation of witnesses
The special counsel can also ask for his jurisdiction to be widened.
He will not be supervised on a day-to-day basis but the attorney general, or in this case Mr Rosenstein, will be able to request an explanation for "any investigative or prosecutorial step" and can decide that any such action does not need to go ahead.
He must notify Congress if such a decision is made.
Who will work for him?
The justice department will provide staff that can work for Mr Mueller, and he can also ask for specific people from both inside and outside the department.
He will have to propose a budget in the next 60 days - and update it annually.
Who can get rid of him?
As acting attorney general in this matter, only Mr Rosenstein can fire him, and for the following reasons: misconduct, dereliction of duty, incapacity, conflict of interest or for other "good cause", including violation of department policies.
However, President Trump could replace his deputy attorney general at any time.
Will the public get to read his report?
Not necessarily. When a special counsel's work ends, a confidential report must be provided to the attorney general explaining the decision to lay charges or not.
The attorney general then has to notify Congress and can decide if the report should be made public.
What's the difference between a special counsel and a special prosecutor?
The term "special prosecutor'" in the US context harks back to the Watergate scandal, when President Nixon's attorney general appointed Archibald Cox to lead an independent investigation.
But there was actually no law defining and regulating such an appointment, which allowed Mr Nixon to later fire Mr Cox. Only later, in 1978, was the Ethics in Government Act passed, which defined the circumstances under which an "independent counsel" could be appointed.
This role actually had more independence from the attorney general than the current special counsel position.
But the legal provision regulating it was allowed to expire in 1999 after the controversy of independent counsel Kenneth Starr's wide-ranging investigation into President Bill Clinton, which started as an inquiry into the Whitewater land deal allegations but ended up providing details of his sexual relationship with White House intern Monica Lewinsky.
So the titles "independent counsel" or "special prosecutor" no longer exist.
Instead the justice department has regulations allowing an outside "special counsel" to be appointed to investigate a person or matter when it might present a conflict of interest for the department or under other "extraordinary circumstances".