Washington(CNN) White House senior policy adviser Stephen Miller has been interviewed as part of special counsel Robert Mueller's Russia probe, according to sources familiar with the investigation.

The interview brings the special counsel investigation into President Donald Trump's inner circle in the White House. Miller is the highest-level aide still working at the White House known to have talked to investigators.

Miller's role in the firing of FBI Director James Comey was among the topics discussed during the interview as part of the probe into possible obstruction of justice, according to one of the sources.

Special counsel investigators have also shown interest in talking to attendees of a March 2016 meeting where foreign policy adviser George Papadopoulos said that he could arrange a meeting between Trump and Russian President Vladimir Putin through his connections. Miller was also at the meeting, according to a source familiar with the meeting.

Papadopoulos was recently charged with lying to the FBI about Russian contacts he had during the campaign.

Earlier this year, Miller assisted Trump in writing a memo that explained why Trump planned to fire Comey, according to sources familiar with the matter. Eventually that memo was scrapped because of opposition by White House counsel Don McGahn, who said its contents were problematic, according to The New York Times. The Comey dismissal letter -- drafted during a May weekend at Trump's golf club in Bedminster, New Jersey -- has also drawn interest from the Mueller team. Sources tell CNN that White House senior adviser Jared Kushner, who was also in New Jersey that weekend, did not oppose the decision to fire Comey. CNN has reported the special counsel's team is asking questions in interviews with witnesses about Kushner's role in Comey's firing.

The Times reported in September that the Justice Department had turned over a copy of the letter, which was never sent, to special counsel Robert Mueller. That memo, according to a source, was very similar to a letter written by Deputy Attorney General Rod Rosenstein that was cited as the reason for firing Comey. Rosenstein's letter criticized Comey's handling of the investigation into Hillary Clinton's private email server.

But just days after the firing, Trump said he considered the Russia probe in his decision to fire Comey.

"In fact when I decided to just do it, I said to myself, I said 'you know, this Russia thing with Trump and Russia is a made-up story, it's an excuse by the Democrats for having lost an election that they should have won'," Trump said about his decision to fire Comey in a May interview with NBC News.

Miller was also an early member of Trump's campaign staff, leaving his role as Sessions' communications director in the Senate to join Trump in January 2016.

Mueller's team has also talked to key former aides including former White House chief of staff Reince Priebus and former White House press secretary Sean Spicer. They have also interviewed National Security Council chief of staff Keith Kellogg. CNN has previously reported Mueller is also seeking to interview other White House staff including McGahn, communications director Hope Hicks and Kushner communications aide Josh Raffel.

The special counsel's office declined to comment. Miller did not respond to a request for comment.

CORRECTION: This story has been updated to correct Josh Raffel's title and the spelling of his name.