Fired FBI Director James Comey is well-known among his former colleagues as a prolific memo-writer, opening the possibility that more documents related to discussions between Comey and President Trump could emerge.

Comey's notes and memos already were being sought by Congress, and could be considered a resource as well for the FBI probe now being overseen by newly appointed special counsel Robert Mueller.

A statement from the Justice Department said he would oversee the probe "of Russian government efforts to influence the 2016 presidential election and related matters."

Ahead of that appointment, The New York Times first reported that Comey in February wrote a memo that Trump had asked him to shut down the investigation into former National Security Adviser Michael Flynn. The memo's existence was confirmed by Fox News on Wednesday.

One Comey confidant told Fox News that the former FBI chief had made a "habit of memorializing important discussions he had with senior officials" over his decades-long law enforcement career. The source did not confirm the existence of more memos, but said it "stands to reason" that Comey made other notes about meetings with Trump.

Comey's memo was made public weeks after the FBI interviewed Flynn regarding his contacts with the Russia ambassador to the United States and after the acting attorney general, Sally Yates, warned the White House that Flynn had misled them about those conversations and could be vulnerable to blackmail.

Flynn was forced to resign Feb. 13, the day before the conversation between Trump and Comey depicted in the memo.

Comey was abruptly fired May 9 and learned of the dismissal as he was giving a talk in Los Angeles. While the White House initially cited a Justice Department recommendation and Comey's very public handling of the Clinton email investigation as reasons, those explanations quickly shifted.

Following the memo's release, some congressional Republicans criticized Comey for not reporting the conversation with Trump to his superiors at the Justice Department. However, a former U.S. government official told Fox News, "That's not how it works."

"Comey was conducting an investigation as to whether President Trump was colluding with the Russian government," the official said. "He’s not going to stop that in order to blow the whistle on a smaller crime (obstruction of justice)."

The official also said that "the FBI, or DOJ, would never indict a sitting president.

"You gather the information and you pass it on to Congress for what could end up in impeachment proceedings," the official said, "and that passing of info to Congress is what you’re starting to see now."

Trump suggested in a statement after the appointment of Mueller that the probe would exonerate his team.

"As I have stated many times, a thorough investigation will confirm what we already know – there was no collusion between my campaign and any foreign entity," Trump said. "I look forward to this matter concluding quickly. In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country."

Acting FBI Director Andrew McCabe told Congress last week the investigation is "highly significant" and said Comey's dismissal would do nothing to impede the probe.

Fox News' Jake Gibson and the Associated Press contributed to this report.