Washington (CNN) Special counsel Robert Mueller's team interviewed former FBI Deputy Director Andrew McCabe and asked about the firing of FBI Director James Comey, a source briefed on the matter confirmed to CNN.

The source would not say when the interview, first reported by Axios, occurred.

Mueller also has memos written by McCabe documenting his conversations with President Donald Trump, a person familiar with the matter told CNN.

The memos also detail what Comey told McCabe about his own interactions with Trump while he was FBI director, the source said, and are seen as a way to corroborate Comey's account in Mueller's probe on Russian interference in the 2016 election.

The Associated Press first reported on the existence of the memos and that the special counsel's office has them.