(RNN) - Retired Lt. Gen. Michael Flynn, the president's national security adviser, has resigned.
Retired Lt. Gen. Keith Kellogg has been named acting national security adviser. Kellogg is a decorated veteran of the Army, including two tours in Vietnam. Kellogg, retired Gen. David Petraeus, and former Vice Admiral Bob Harward are among the candidates to replace Flynn, CNN is reporting.
On Friday, The Washington Post reported that Flynn addressed sanctions against Russia in a call with Russia's ambassador to the U.S.
The report contradicted Trump officials, including Vice President Mike Pence, who vouched for Flynn in a television interview based off information given to him by Flynn.
Flynn later apologized to Pence. Flynn's resignation letter addressed the impact of the question of his discussion of Russia sanctions with Russian officials.
"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 wrote in a resignation letter obtained by media outlets. "I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way."
Flynn admitted to some administration officials that sanctions may have come up in the calls, which came in at the same time that the Obama administration was slapping penalties on Russia for election-related hacking.
The Associated Press reported that Justice Department officials told the White House weeks ago that Flynn's contacts could leave him in a "compromised position."
Flynn was chosen by Donald Trump for the national security adviser position shortly after the election, and he bypassed Senate confirmation, taking on the job immediately after inauguration.
He was one of Trump's biggest backers and spoke on his behalf at the Republican National Convention.
Flynn worked in the Obama Administration and was considered as a running mate for Trump before the billionaire selected Mike Pence.
The retired three-star general, a registered Democrat, served as Trump's adviser on Islamic State, Iran and the military. His 2012 to 2014 tenure at the Defense Intelligence Agency, which functions as the main foreign U.S. military espionage organization, was troubled, and he butted heads with the Obama administration before being forced out.
A 33-year veteran of the U.S. Army, Flynn served several tours in Iraq and Afghanistan. He was commissioned in 1981 as a second lieutenant in military intelligence.
While head of the Defense Intelligence Agency, he was also commander of the Joint Functional Component Command for Intelligence, Surveillance and Reconnaissance and the chair of the Military Intelligence Board.
Flynn advised Gen. Stanley McChrystal in Iraq and Afghanistan, according to his Leading Authorities speaking biography. He also served as the assistant director of National Intelligence for Partner Engagement; the director of intelligence for the Joint Special Operations Command; and the senior intelligence adviser for Chairman of the Joint Chiefs Admiral Mike Mullen.
He held assignments supporting the Cold War; operations Urgent Fury, Uphold Democracy, Desert Shield, Desert Storm, Desert Thunder II, Desert Fox, Enduring Freedom, Iraqi Freedom and other classified operations.
Flynn is the author of The Field of Fight: How We Can Win the Global War Against Radical Islam and Its Allies.
He graduated from the University of Rhode Island in 1981.