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.
Also on Saturday, the president's lawyer John Dowd issued a statement saying it was time for the special counsel's investigation to end.
Why was McCabe fired?
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.
Attorney General Jeff Sessions said the "extensive and fair investigation" had concluded that McCabe "made an unauthorised disclosure to the news media and lacked candour - including under oath - on multiple occasions".
Although the decision to fire McCabe was made by Mr Sessions, Trump had criticised him for months.
He has publicly pointed to donations that McCabe's wife, a Democrat, received from a Clinton ally when she ran unsuccessfully for the state Senate in 2015 as evidence that Mr McCabe was politically biased.
In December Trump tweeted: "FBI Deputy Director Andrew McCabe is racing the clock to retire with full benefits. 90 days to go?!!!"
He welcomed the news of his dismissal almost immediately after Sessions announced it, calling the move a "great day for democracy".
Trump's tweet about the firing provoked an angry response from former CIA director John Brennan, who implied McCabe was being made a scapegoat.
News that McCabe had kept records of his conversations with Trump and FBI Director at the time James Comey emerged on Saturday.
US media say the memos will support Comey's account of the circumstances of his dismissal last May.
Comey has testified that 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.
In a statement responding to his firing on Friday, McCabe vehemently denied wrongdoing.
"I am being singled out and treated this way because of the role I played, the actions I took, and the events I witnessed in the aftermath of the firing of James Comey," his response said.
The statement alleges that the justice department report recommending his firing was "accelerated" after he indicated that he would corroborate Comey's version of events, reports BBC.
On Saturday Mr Comey - who is due to release a book - said Americans would soon be able to judge for themselves "who is honourable or not" after the president again criticised him on Saturday.
The president continued posting about McCabe's departure on social media, saying there had been "leaking, lying and corruption" at the FBI, as well as the defence and state departments.