'Single greatest witch hunt of a politician in American history:' Trump

He is entitled to a budget, can request new or additional staff and will have the same authority as high-ranking Justice Department lawyers, including the ability to prosecute any crimes he uncovers.

Republicans have largely stood behind Trump as the FBI and congressional investigations into Russia's election meddling intensified.

"That's the nature of these investigations - that once you begin to look at one set of issues, one quickly realizes that they are connected to others", said James Davis, an American scholar and dean of the School of Economics and Political Science at St. Gallen University in Switzerland.

Who is Robert Mueller anyway?

During his tenure as director of the Federal Bureau of Investigation, from 2001 to 2013, he served both Republican and Democratic administrations, overseeing a shake-up of a huge bureaucracy blamed for missing evidence that could have prevented the September 11, 2001 attacks, and earning high respect from both parties. Gohmert told Fox News.

Deputy Attorney General Rod Rosenstein, who appointed the special counsel, was due to brief US senators later on Thursday about Comey's removal.

The White House says an investigation will prove there was no collusion.

The White House Wednesday night was aiming for a measured response to Mueller's appointment and ended the night relatively pleased at the muted approach they believed the President put forward. Comey gave Fitzgerald complete discretion to conduct the investigation, bolstering the special counsel's independence.

There had been a growing call from all political parties for a special prosecutor to take over the Russian Federation investigation.

Q. Could a special counsel investigate things beyond simply the Russian Federation connection?

Trump's Twitter tirade comes as Reuters reports Michael Flynn and other advisers to Donald Trump's campaign were, indeed, in contact with Russian officials and others with Kremlin ties in at least 18 calls and emails during the last seven months of the 2016 presidential race.

After announcing Comey's firing, the White House publicly released a memo from Rosenstein saying Comey was sacked in large part because of his mishandling of the investigation into Hillary Clinton's use of a private email server as secretary of state.

That justification was viewed with skepticism by many until Trump said in an interview days later he fired Comey because of his continued probe into Russian interference and collusion in the 2016 campaign and would have fired him regardless of what the Department of Justice recommended.

"Every effort has been made that he can operate as independently as possible", said Davis.

The President's adoption of the role of a victim has been a potent political weapon before and he has frequently used it to position himself against the Washington establishment, including the media which he uses as a foil in the absence of an easily identifiable political enemy, like Hillary Clinton.

"It's a very risky situation if the president has something to hide", said Davis, because Mueller will follow wherever the investigation will take him. But everyday voters aren't convinced that the investigation is worthwhile.

Rosenstein, who at his confirmation hearing would not commit to a special counsel appointment and who inherited the probe following Sessions' recusal, seemed to acknowledge the public outcry by calling Mueller's selection "necessary in order for the American people to have full confidence in the outcome". How long the investigation takes then depends on what Mueller does or does not find.