The F.B.I. had been examining Mr. Flynn’s phone calls as he came under growing questions about his interactions with Russian officials and his management of the National Security Council. The blackmail risk envisioned by the Justice Department would have stemmed directly from Mr. Flynn’s attempt to cover his tracks with his bosses. The Russians knew what had been said on the call; thus, if they wanted Mr. Flynn to do something, they could have threatened to expose the lie if he refused.
The Justice Department’s warning to the White House was first reported on Monday night by The Washington Post.
In addition, the Army has been investigating whether Mr. Flynn received money from the Russian government during a trip he took to Moscow in 2015, according to two defense officials. Such a payment might violate the Emoluments Clause of the Constitution, which prohibits former military officers from receiving money from a foreign government without consent from Congress. The defense officials said there was no record that Mr. Flynn, a retired three-star Army general, filed the required paperwork for the trip.