Former acting attorney general Sally Q. Yates warned the White House that national security adviser General Michael Flynn may be vulnerable to Russian blackmail, the Washington Post reported on Monday.
Yates had delivered the message late last month amid worries about Flynn's communication with the Russian ambassador in Washington, according to the report, which cited unnamed current and former U.S. officials.
It wasn't clear, the Post reported, what White House Counsel Donald McGahn had done with Yates' information.
Yates was fired for opposing Trump's temporary entry ban for people from seven mostly Muslim nations.
Flynn had told Vice President Mike Pence he had not discussed U.S. sanctions against Russia with Russian officials in the weeks before Trump took office on Jan. 20, prompting Pence to defend him in subsequent television interviews.
In recent days, Flynn has acknowledged he might have discussed sanctions with the Russians but could not remember with 100 percent certainty, which officials said had upset Pence, who felt he had been misled. Officials said Flynn apologized to Pence twice, including in person on Friday.
"The president is evaluating the situation. He is speaking to ... Vice President Pence relative to the conversation the vice president had with General Flynn and also speaking to various other people about what he considers the single most important subject there is, our national security," White House spokesman Sean Spicer said.
—Reuters contributed to this report.