Specifically, Mueller will explore the possibility that Trump obstructed justice.
The special counsel is following two major lines of investigation, said one US official familiar with the rough outlines of Mueller's probe who spoke on condition of anonymity.
He goes on to list several "key points", the first and foremost being: "The president never obstructed justice".
"Whoever leaked [news of the obstruction investigation] was obviously reading that he was thinking about giving Mueller the boot", an official told the Daily Beast.
One exchange of particular interest to Mueller involves a March 22 White House briefing when then-newly confirmed Coats, together with other government officials, were asked to leave the room, except for Coats and CIA Director Mike Pompeo.
Attorney General Jeff Sessions, a close aide of President Donald Trump, has vehemently denied any role in Russia's alleged meddling in the 2016 United States polls and dismissed accusations that he discussed such an effort with Russian officials as an "appalling and detestable lie".
On Thursday, Russian President Vladimir Putin said Comey had presented no evidence to prove that Moscow meddled in the USA election, adding that Washington had tried to influence Russian elections "year after year", he said.
That isn't true anymore, according to reporting by The Washington Post on Wednesday night.
It comes a day after lawmakers questioned Justice Department officials about the probe and Mueller's independence.
Comey said at the Senate hearing that he had an associate leak detailed memos about his conversations with Trump in part because he felt so uncomfortable with Trump's conversations with him and hoped to push the Justice Department into appointing a special counsel.
Two months after Barack Obama's inauguration, the White House's top lawyer, Greg Craig, issued a memorandum to the attorney general and other department heads laying out what he said were "procedures. created to ensure that this Administration acts responsibly and consistently with respect to White House confidentiality interests, with due regard for the responsibility and prerogatives of Congress".
During his questions, Cotton noted there's still no public evidence that anyone in the Trump campaign actively colluded with Russian Federation during the 2016 election.
Sessions declined to answer those questions and Democrats made clear they objected.
Senate Intelligence Committee Chairman Richard Burr, R-N.C., and Sen.
Mueller also won votes of support Tuesday from the top two Republicans in Congress, House Speaker Paul Ryan of Wisconsin and Senate Majority Leader Mitch McConnell of Kentucky, both of whom said they have confidence in him.