‘It would have been impossible for me to know this’

Mr. Trump was warned just after the election and in the first days of his presidency that keeping on Mr. Flynn, who was a campaign adviser, could be troublesome. And during the transition, Mr. Flynn told a top Trump aide that he was under federal investigation.

He was first warned about Mr. Flynn two days after the election, when Mr. Trump met in the Oval Office with President Barack Obama, who cautioned his successor not to give Mr. Flynn a senior post on his national security team. (Mr. Obama had fired Mr. Flynn in 2014 as head of the Defense Intelligence Agency after repeated clashes over his management.)

But even if Mr. Trump did not interpret Mr. Obama’s comments as a warning, Mr. Flynn himself told the transition team’s lawyer, Donald F. McGahn II, weeks before inauguration that he was under federal investigation for paid lobbying work he did secretly for Turkey while he was working on the presidential campaign.

Then, six days after Mr. Trump’s inauguration, the acting attorney general, Sally Q. Yates, told Mr. McGahn, by then the White House counsel, that Mr. Flynn was vulnerable to blackmail because he misled the vice president about conversations he had with the Russian ambassador to the United States during the transition period.

‘With me being one of two people who would become president’

Mr. Trump’s advisers have complained that the F.B.I. never disclosed to him during the campaign that some of the people closest to him were under investigation for their ties to Russia.