“The president urged me, like he has everyone in the administration, to fully cooperate with the special counsel,” Sanders said Friday in an email. “I was happy to voluntarily sit down with them.”

CNN reported earlier Friday that Mueller’s team had interviewed Sanders, citing sources it didn’t identify, and said it happened about the same time the special counsel interviewed former White House Chief of Staff John Kelly. It isn’t clear when either interview took place. CNN said the White House didn’t initially agree to allow Sanders to talk to Mueller, citing one person familiar with the matter.

Sanders didn’t say what the special counsel asked her. Mueller’s spokesman Peter Carr declined to comment.

Sanders is one of the president’s longest-serving aides in the White House, and worked for Trump when he fired former FBI Director James Comey, and when the president helped his son, Donald Trump Jr., draft an erroneous statement defending a meeting he held with a Russian lawyer during the 2016 campaign.

Both events have attracted Mueller’s attention as part of an investigation into whether Trump sought to obstruct the probe into his campaign’s ties to Russia.

Sanders could also help Mueller determine whether Trump had corrupt intent in his many tweets and statements attacking Mueller’s investigation and law enforcement in general, or whether the president sought to intimidate witnesses.

Mueller may also be interested in statements Sanders has made about the investigation and whether they were directed by Trump.

The press secretary has frequently commented on developments in Mueller’s probe. Last month, for example, following the arrest of Trump associate Roger Stone, Sanders said on CNN that “the question and the big thing that the Mueller investigation is supposed to center on is whether or not the president in some outrageous way colluded with Russia and the answer to that is no.”

And in an August press briefing, Sanders declared that Mueller’s “entire investigation is based off a dirty, discredited dossier that was paid for by an opposing campaign and had a lot of corruption within the entity which was overseeing it, which was Peter Strzok, James Comey, Andrew McCabe.”