The Justice Department tapped former FBI director Robert Mueller to be a special counsel overseeing the Russia investigation, the department announced Wednesday.

The news comes as President Trump and his administration grapple with the fallout from explosive revelations earlier this week that now-fired FBI director James Comey kept notes of a February meeting indicating Trump asked him to close the agency's investigation into former national security adviser Michael Flynn.

The memo, on the heels of Trump's abrupt firing of Comey last week, fueled accusations by lawmakers of possible obstruction of justice – and calls for an independent prosecutor to oversee the FBI's ongoing counterintelligence probe into possible collusion between Trump campaign associates and Russia during the presidential campaign.

"I determined that it is on the public interest for me to exercise my authority and appoint a special counsel to assume responsibility for this matter," Deputy Attorney General Rod Rosenstein said Wednesday. "My decision is not a finding that crimes have been committed or that any prosecution is warranted."

Based on the "unique circumstances," Rosenstein said, "the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command."

After the Justice Department's announcement, Trump on Wednesday night denied any collusion between his campaign and Russia and called for a quick end to the investigation.

"As I have stated many times, a thorough investigation will confirm what we already know – there was no collusion between my campaign and any foreign entity," Trump said in a short statement. "I look forward to this matter concluding quickly. In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country."

The new special counsel, Mueller, served as FBI director from 2001 to 2013 under both the George W. Bush and Obama administrations.

Mueller, who preceded Comey as FBI director, was the longest serving director since J. Edgar Hoover. He served two additional years beyond his 10-year term, to ensure stability during a transition period in President Obama's national security team.

In his special counsel role, Mueller assumes all the powers of a federal prosecutor – including subpoena authority.

"I have determined that a special counsel is necessary in order for the American people to have full confidence in the outcome," Rosenstein said. "Our nation is grounded on the rule of law and the public must be assured that government officials administer the law fairly. Special Counsel Mueller will have all appropriate resources to conduct a thorough and complete investigation, and I am confident that he will follow the facts, apply the law and reach a just result."

The authority to appoint a special counsel fell to Rosenstein, after Attorney General Jeff Sessions recused himself from the Russia case because of his pre-election contacts with Russian ambassador Sergey Kislyak, which Sessions did not disclose during his Senate confirmation hearing.

Mueller, who joined law firm WilmerHale in 2014, will resign to avoid any conflicts of interest with firm clients or attorneys, the Justice Department said.

At the White House, administration officials Wednesday evening could be seen racing in and out of West Wing offices, formulating a response to the new development.

White House officials such press secretary Sean Spicer had said as recently as Tuesday there was no need for a special counsel.

Yet key Republicans lawmakers immediately welcomed the announcement. "Mueller is a great selection. Impeccable credentials. Should be widely accepted," tweeted Rep. Jason Chaffetz, R-Utah, chair of the powerful House Oversight and Government Reform committee.