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.
Attorney General Jeff Sessions fired McCabe on Friday night, just over 24 hours before his 50th birthday and the date he was set to retire and begin receiving his anticipated pension, over accusations that McCabe directed FBI officials to speak to the media about an investigation tied to the Clinton Foundation and misled investigators about his actions. Following his firing Friday, McCabe told CNN in an interview that he had four interactions with the President last May, while he was acting FBI director.
McCabe revealed that he had three in-person interactions and one phone call with Trump, in which the President berated him each time about his wife's failed Virginia Senate campaign.
It is unclear exactly what is in McCabe's memos and if he memorialized every interaction he had with the President. McCabe did not keep a copy of his memos after turning them over to Mueller, the source confirmed. A spokesman for the special counsel's office declined comment.
"In May, when Director Comey was fired and I had my own interactions with the President, he brought up my wife every time I ever spoke to him," McCabe told CNN. "Of course, I disagreed with him."
McCabe also confirmed that the President asked him who he voted for in the 2016 election, which was reported back in January and which Trump denied.
The former No. 2 official at the FBI told CNN that Trump did not bring up the agency's investigation into Russia meddling in the 2016 election.
Trump responded to the news that McCabe kept memos by tweeting that he only would have written them "to help his own agenda."
"Spent very little time with Andrew McCabe, but he never took notes when he was with me," the President tweeted Sunday morning. "I don't believe he made memos except to help his own agenda, probably at a later date. Same with lying James Comey. Can we call them Fake Memos?"
He also launched into an attack on Mueller's team, alleging possible political bias.
"Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans?" Trump wrote. "Another Dem recently added ... does anyone think this is fair? And yet, there is NO COLLUSION!"
Comey revealed last year that he had kept memos while he was still FBI director about meetings and conversations he had with the President. Comey said his memos detailed conversations in which Trump asked Comey to pledge loyalty to him, state publicly that the FBI was not investigating Trump himself, and urged Comey to "let this go," referring to the then-investigation involving Trump's former national security adviser Michael Flynn.
Trump has denied several of Comey's claims.
During his testimony on Capitol Hill last June, Comey acknowledged orchestrating the leak of his memos to the media a few days after he was fired in hopes that the Justice Department would appoint a special counsel.
The former FBI director's memos have also been handed over to the special counsel.