Ex-FBI deputy director Andrew McCabe has given memos about conversations he had with President Donald Trump to an inquiry into alleged Russian meddling in the 2016 election, US media say.
They say the memos could support allegations that the president sought to obstruct justice.
Mr McCabe was fired from the FBI on Friday following an internal inquiry. Mr Trump had accused him of bias.
The president has also dismissed the Russia investigation as a "witch hunt".
The investigation is led by special counsel Robert Mueller, himself a former FBI director. He has so far indicted 19 people.
Mr McCabe had been under internal investigation by the FBI and had already stepped down from his deputy post in January pending the review.
He was sacked just two days short of his 50 birthday on Sunday, when he was expected to retire with a federal pension.
News that Mr McCabe had kept records of his conversations with Mr Trump and FBI Director at the time James Comey emerged on Saturday.
US media say the memos will support Mr Comey's account of the circumstances of his dismissal last May.
Mr Comey has testified that Mr Trump had asked him for his "loyalty" and requested he drop an inquiry into his former National Security Advisor Michael Flynn.
He too has said he kept contemporaneous notes of his dealings with the president.