Comey himself said when he testified before the Senate last week that he was "sure" that Mueller's investigation would look into a potential obstruction of justice case against President Donald Trump.
The Washington Post reported late Wednesday that special counsel Robert Mueller is seeking interviews with three Trump administration officials who weren't involved in Trump's campaign.
Comey told the Senate Intelligence Committee on June 8 he believed Trump had directed him in February to drop an FBI probe into former national security adviser, Michael Flynn, that was part of the broader Russian Federation investigation.
According to the Washington Post, the decision by the special counsel to investigate Trump marks a major milestone in the FBI investigation that has lasted nearly a year now.
The Post report cited anonymous sources who were briefed on requests made by investigators.
He added that he came to know about it from his sources, but the President did not tell him personally about it. Trump was considering it as an option, reported Fox News. The two sides have been discussing "deconfliction" - or how the Senate investigation and the federal probe will share information and not step on each others' toes.
The White House wouldn't deny or confirm the report and said, "only the president or his attorneys are authorized to comment".
Trump vehemently denies any collusion between himself or any of his associates and Russian Federation.
Trump has repeatedly denied any coordination with Russian Federation, and has assailed the probe as a "witch hunt".
The NSA said in a statement: "NSA will fully cooperate with the special counsel".
It comes a day after lawmakers questioned Justice Department officials about the probe and Mueller's independence.
"I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct", Comey said.
Mueller has also gathered the memos which Comey created following the now-infamous meeting in which he claims Trump asked him to end his investigation into Flynn.