A former FBI Director has been appointed special prosecutor to take over the Justice Department's investigation into possible Russian interference in the 2016 election.
Robert Mueller will probe any potential ties Trump campaign officials may have had with Russians.
Deputy Attorney General Rod Rosenstein was charged with appointing the special prosecutor after Attorney General Jeff Sessions recused himself from the Russia probe because he had lied about meeting with Russian officials.
The decision by Mr Rosenstein follows a week of intensifying pressure on the Justice Department to make sure that its investigation into the White House is independent.
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Mr Rosenstein was identified by the White House as having played an influential role in Mr Comey's firing, though Donald Trump later said that he would have fired him with or without a good reason being presented from the deputy attorney general.
"My decision is not a finding that crimes have been committed or that prosecution is warranted," Mr Rosenstein said. "I have made no such determination."
Mr Trump responded to the appointment of a special prosecutor by saying the investigation will confirm there was "no collusion" with the Kremlin.
"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," he said in a statement released by the White House.
"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."
Mr Trump's surprising decision last week to fire former FBI Director James Comey - and subsequent reports that the President had allegedly asked Mr Comey to stop the FBI's Russia probe - sent Washington into a frenzy and resulted in several members of Congress calling for special prosecutors to look into the matter.
Pundits on cable television hailed the special prosecutor appointment as a major development in and those who have worked with Mr Rosenstein - including former US attorney Preet Bharara, who was fired by Mr Trump - said that Mr Mueller is one of the best choices possible to run the investigation.
"Based on the unique circumstances, 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," Mr Rosenstein said.
Mr Mueller is a respected figure in American law enforcement circles. He headed the FBI from 2001 to 2013 and served under Presidents George W Bush and Barack Obama.
Mr Obama asked him to stay on at the FBI past the usual 10 year mark.
As special prosecutor, Mr Mueller would still be answerable to Mr Rosenstein, who is answerable to the president, but he will have greater autonomy to run the investigation than if he were usual US attorney.
Separate investigations are ongoing in Congress.