WASHINGTON (Reuters) – President Donald Trump’s national security adviser, Michael Flynn resigned late on Monday under scrutiny over whether he discussed the possibility of lifting U.S. sanctions on Russia before Trump took office.

Flynn submitted his resignation hours after Trump said through a spokesman that he was reviewing the situation and talking to Vice President Mike Pence. Flynn had promised Pence he had not discussed sanctions with the Russians but it was later discovered that the subject had come up.

“Unfortunately, because of the fast pace of events, I inadvertently briefed the vice president-elect and others with incomplete information regarding my phone calls with the Russian ambassador. I have sincerely apologized to the president and the vice president, and they have accepted my apology,” Flynn said in his resignation letter.

Retired General Keith Kellogg, who has been chief of staff of the White House National Security Council, was named the acting national security adviser while Trump determines who should fill the position.

Retired General David Petraeus, a former CIA director, is under consideration for the position, a White House official said.

Flynn’s resignation came after it was reported that the Justice Department warned the White House weeks ago that Flynn could be vulnerable to blackmail for contacts with Russian officials before Trump took power on Jan. 20.

A U.S. official confirmed a Washington Post report that Sally Yates, the then-acting U.S. attorney general, told the White House late last month that she believed Flynn had misled them about the nature of his communications with the Russian ambassador to the United States.

She said Flynn might have put himself into a compromising position, possibly leaving himself vulnerable to blackmail, the official said. Yates was later fired for opposing Trump’s temporary entry ban for people from seven mostly Muslim nations.

Flynn, a retired U.S. Army lieutenant general, was an early supporter of Trump and shares his interest in shaking up the establishment in Washington. He has frequently raised eyebrows among Washington’s foreign policy establishment for trying to persuade Trump to warm up U.S. relations with Russia.

(Additional reporting by John Walcott; Editing by Peter Cooney, Robert Birsel)