Special counsel Robert Mueller's office has interviewed White House press secretary Sarah Huckabee Sanders as part of his investigation into Russian interference in the 2016 presidential election.

Sanders told CNN on Friday that she sat down with Mueller's team voluntarily.

"The President urged me, like he has everyone in the administration, to fully cooperate with the special counsel. I was happy to voluntarily sit down with them," she said.

Sanders is one of several current and former Trump administration officials who are known to have been interviewed as part of Mueller's investigation, which is also looking into whether members of Trump's campaign colluded with Russia.

CNN reported that the substance of Sanders's interview is unknown, but that Mueller is likely interested in public statements made by Sanders and other administration officials throughout the investigation.

Sanders has made numerous public statements in defense of the president while serving as his top spokesperson, including claiming that the president "certainly didn't dictate" his son Donald Trump Jr.'s statement about a 2016 meeting in Trump Tower with a Russian lawyer who promised damaging information about Hillary Clinton.

That statement's truthfulness was thrown into question after The New York Times published a memo from the president's own legal team last year contradicting Sanders's statement.

Mueller's investigation is thought to be in its final stages, as former acting Attorney General Matthew Whitaker told reporters last month that the probe was "close to being completed."

The president has repeatedly attacked the probe as a "witch hunt" into his administration and personal businesses, even as the the investigation has ensnared former Trump campaign officials and Russian nationals with criminal charges.

- John Bowden contributed