Top justice department official Rod Rosenstein, who oversaw the inquiry into alleged Russian meddling in 2016, is planning to quit, US media reports.
The deputy attorney general reportedly made the decision to step down and was not forced out by the White House.
But he has been a frequent target of President Donald Trump's criticism.
Mr Rosenstein had reportedly considered resigning last fall after a report emerged that he suggested secretly recording Mr Trump.
The no 2 US Department of Justice official plans to stay on until the confirmation of attorney general nominee William Barr, sources say.
Mr Rosenstein has yet to comment on the reports.
Senate hearings for Mr Barr's confirmation are scheduled to begin on 15 January.
If confirmed, Mr Barr will succeed acting attorney General Matthew Whitaker, who replaced former Attorney General Jeff Sessions after he was fired by Mr Trump.
There is no set timeline for Mr Rosenstein's departure, according to US media.
CBS News reports that Mr Rosenstein will depart in the next few weeks, after Mr Barr is expected to take over, in order to ensure a smooth transition.
According to Fox News and ABC News, Mr Rosenstein had long planned to leave after two years as deputy attorney general.
White House Press Secretary Sarah Sanders told Fox News on Wednesday that she knew "the deputy attorney general has always planned to roughly stay around two years".
"My guess is that he is making room for the new attorney general to build a team that he wants around him," she added.
What's the background?
Former Attorney General Jeff Sessions recused himself from overseeing Special Counsel Robert Mueller's inquiry into suspected Russian meddling in the 2016 presidential election.
He then handed off oversight to his deputy, putting Mr Rosenstein in the spotlight and leading to criticism from Republicans in Congress as well as Mr Trump.
But after Mr Session was ousted in November, his replacement Matthew Whitaker took over the investigation, refusing to recuse himself despite his previous public criticism of the probe. Mr Rosenstein continued to help supervise the inquiry.
If Mr Barr is confirmed, he is likely to take over the role of overseeing the Russia investigation.
On Wednesday, Mr Barr travelled to Capitol Hill to meet with members of the Senate.
Republican Senator Lindsey Graham said after the two men met, Mr Barr had disagreed with the president's assertion that the Russia probe is "a witch hunt".
White House adviser Kellyanne Conway said Mr Rosenstein will not immediately leave so that he can "help with the transition with bringing the new attorney general in.
"We hope that that happens relatively soon," she said, adding Mr Rosenstein that has "done a great job".
Have Trump and Rosenstein feuded?
In September the New York Times reported, citing anonymous sources, that Mr Rosenstein had discussed recruiting cabinet members to invoke the 25th amendment of the US constitution, which provides for the removal of a president if he is deemed unfit for office.
The newspaper also reported that he suggested secretly recording Mr Trump to expose White House turmoil and dysfunction.
Mr Rosenstein denied the report, which he said was "based on anonymous sources who are obviously biased against the department".
A US Department of Justice official later told the BBC that someone "who was in the room" and heard Mr Rosenstein's secret recording suggestion had considered it to be a joke.
In November, Mr Trump retweeted an image of Mr Rosenstein behind prison bars.
The post suggested that he had committed "treason".
Mr Trump has repeatedly railed against the special counsel's probe into claims that Russia sought to influence the election in Mr Trump's favour.
He has accused the investigation of being led by "Democrat loyalists".
Mr Rosenstein has spent his career at the Department of Justice, and most recently served for 12 years as the US Attorney General for the state of Maryland before beginning his current role in April 2017.