WASHINGTON — Former FBI official Andrew McCabe memorialized his interactions with President Trump in contemporaneous memos, a person familiar with the case said, and they could become a key piece of evidence in special counsel Robert Mueller’s probe.

Mueller has been investigating, among other things, whether Trump obstructed justice in his interactions with top law enforcement officials, including McCabe and his former boss, FBI Director James Comey. Comey also kept memos documenting his interactions with Trump, which Mueller already was reviewing.

The memos could help bolster McCabe’s credibility, insulating him from allegations that he misstated or misremembered his interactions with Trump. On Friday, McCabe was fired from the FBI, about 26 hours before he was set to retire, over allegations from the Justice Department’s inspector general that he authorized the disclosure of information to a reporter about an ongoing criminal investigation, then misled investigators about it. McCabe disputes that he misled anyone or did anything wrong.

McCabe had been the FBI’s No. 2 official until earlier this year, when he stepped down after FBI Director Christopher Wray was briefed on the inspector general’s findings. He had remained an FBI employee until Friday when Attorney General Jeff Sessions, acting on a recommendation from the FBI’s disciplinary office, fired him over the allegations.

It was not immediately clear which interactions with Trump the memos detailed, or how specific they were.

McCabe said Friday night Comey was “aware of the interaction” when he authorized two other FBI officials to speak with a reporter. Comey wrote on Twitter just minutes after Trump’s tweet, “Mr. President, the American people will hear my story very soon. And they can judge for themselves who is honorable and who is not.”