The Justice Department has appointed former FBI director Robert Mueller as a special counsel to oversee a federal investigation into potential co-ordination between Russia and the Trump campaign during the 2016 presidential election.

The appointment on Wednesday came amid a growing Democratic outcry for someone outside the Justice Department to handle the politically charged investigation.

READ MORE: Trump could face impeachment if FBI interference claims are true

It followed the revelation Tuesday that fired FBI director James Comey had written in a memo that Trump, in a February meeting in the Oval Office, had asked him to end an investigation into former national security adviser Michael Flynn. The White House has denied that account.

Mueller was appointed FBI director in 2001 and led the FBI through the aftermath of the September 11 terror attacks. He retired in 2013.

The Justice Department said Mueller had resigned from his job at a private law firm to take the job of special counsel.

The Justice Department declined to explain the decision-making involved in the appointment.

What is known about US investigations into Russian election meddling:

HOW DID INVESTIGATIONS BEGIN?

Former President Barack Obama ordered US intelligence agencies to assess whether Russia intervened in the election after a cyber attack on the Democratic National Committee in July last year and the publication of thousands of personal emails from Hillary Clinton's campaign manager.

WHAT DID INTELLIGENCE AGENCIES FIND?

The CIA and FBI found Russian President Vladimir Putin ordered a campaign to undermine confidence in the US electoral system and affect the outcome with a "clear preference" for Trump to win.

Putin's associates hacked information, paid social media "trolls" and backed efforts by Russian government agencies and state-funded media to sway public opinion.

WHAT HAS TRUMP SAID ABOUT RUSSIA'S ROLE?

Trump has not taken a clear public position but said in July last year "I will tell you this, Russia: if you're listening, I hope you're able to find the 30,000 emails that are missing".

Trump subsequently dismissed reports from US intelligence officials that Russia attempted to intervene in the election on his behalf.

The first time Trump said he accepted the findings of the intelligence agencies was at a January conference ahead of his inauguration. "As far as hacking, I think it was Russia," he said, but then added: "It could have been others also."

HOW MANY US PROBES ARE THERE?

Committees in the House of Representatives and the Senate along with the US Department of Justice and FBI are known to be investigating.

WHAT HAS THE FALLOUT BEEN?

Trump's national security adviser Michael Flynn was fired in February. The White House said he misled Vice President Mike Pence about his contact with Russia's ambassador to the US, Sergei Kislyak, before Trump took office.

Attorney General Jeff Sessions had to recuse himself from involvement in Russia- related probes at the Justice Department because he did not tell Congress of his contacts with Kislyak in 2016.

FBI director James Comey was sacked by Trump on May 9.

WHY WAS COMEY FIRED?

The White House says Comey was fired because of his handling of the Clinton email investigation.

A memo from Deputy Attorney General Rod Rosenstein to Trump recommended Comey's dismissal. It said Comey erred in July last year by announcing the FBI had been examining Clinton's use of a private email server and that the case should be closed without prosecution.

A memo Comey wrote after a February meeting with Trump stated the president had asked him to end the FBI's investigation of Flynn, the New York Times reported.

Trump called Comey a "showboat" and "grandstander" in May.

Comey told the congressional panel in May the idea that he may have affected the election result made him "mildly nauseous".

WHAT HAPPENS NOW?

Former FBI director Robert Mueller has been appointed as a special counsel to oversee the Trump-Russia probe. It comes amid a growing Democratic outcry for someone outside the Justice Department to handle the politically charged investigation.