Former FBI chief Robert Mueller has been put at the helm of the politically-charged investigation into alleged Russian interference in last year's presidential election.
Mueller, who led the Bureau through the aftermath of the September 11 attacks, was appointed special counsel for the inquiry.
But what exactly does that mean?
Here's what you need to know:
- Who is Robert Mueller?
The 72-year-old attorney was the sixth director of the FBI, serving from September 4, 2001 - starting just days before the 9/11 attacks - until September 4, 2013.
The Princeton University graduate served in the Vietnam War and earned medals including the Purple Heart and the Vietnamese Cross of Gallantry.
Since leaving the FBI, he has served as a guest lecturer at Stanford University - where he focuses on cyber security issues - and was a partner at law firm WilmerHale until he resigned to take up the role as special counsel.
His successor at the Bureau was James Comey, who earlier this month was fired from the role and from the Russia investigation, which Mueller will now pick up.
- What will he be doing?
Mueller has been tasked with continuing the investigation into any links between President Donald Trump and Russia, along with "any matters that arose or may arise directly from the investigation", according to the letter appointing him.
This is the investigation about which recently-fired FBI director James Comey told Congress, before he was fired by Trump.
- What is a special counsel and what are their powers?
According to US Justice Department regulations, a special counsel has all the authority of a US attorney, including the ability to initiate investigations, subpoena records and bring criminal charges.
The difference is they are able to choose whether or not they tell the Justice Department what they are up to - though they must notify the attorney general before taking any "significant" actions.
- Could Trump fire Mueller?
No. Special counsels can only be fired by the "personal action" of the attorney general.
However, since attorney general Jeff Sessions has recused himself from the Trump-Russia investigation, it will be his deputy Rod Rosenstein who has the authority to do the firing.
It would require a finding of incapacity, misconduct, or an otherwise "good cause", and he would have to inform Mueller in writing.
Rosenstein is also the one who recommended that James Comey be fired from the FBI.
- Will the outcome of the investigation be made public?
Maybe - but not necessarily.
When a special counsel finishes their investigation, they must give the attorney general a confidential report explaining their decision either to bring charges or to drop the matter.
The attorney general would have to notify Congress of the conclusion, but can decide whether to make that report public.