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.

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