Special counsel Robert Mueller has interviewed White House senior policy adviser Stephen Miller as part of the federal investigation to determine whether the Trump campaign colluded with Russia during the 2016 election.

Miller was present at a March 2016 meeting where George Papadapoulos, a foreign policy adviser to the Trump campaign, said he could coordinate a meeting with Trump and Russian President Vladimir Putin, a source told CNN.

It was reported last week that Papadapoulos pleaded guilty to lying to the FBI about his Russian contacts during the campaign.

Miller helped Trump write a memo that outlined why Trump intended to fire former FBI Director James Comey in May. However, the memo was never used because White House counsel Don McGahn pushed back because the content of the memo was concerning, the New York Times reported.

The memo, which was drafted in Bedminster, N.J., at Trump’s golf club, is under scrutiny by Mueller. The Justice Department reportedly provided a copy of the letter to Mueller. The contents of the letter were akin to the one written by Deputy Attorney General Rod Rosenstein that was used to dismiss Comey, a source told CNN.

Miller is the highest level aide who is still working at the White House to have been interviewed by Mueller’s team.

Former White House officials were already interviewed by Mueller’s team this fall, including former chief of staff Reince Priebus and former press secretary Sean Spicer. Keith Kellogg, who served as interim national security adviser following the ouster of Mike Flynn, also spoke to the Mueller team.

Meanwhile, Hope Hicks, the White House communications director, will reportedly be interviewed by Mueller's team in mid-November.

In September, it was reported that Mueller was interested in interviewing White House aides to see what they knew about the investigation into Flynn, who was fired in February after it was discovered he misled the administration about his communications with Russians.

The White House anticipates Mueller’s investigation will conclude by Thanksgiving.