With his administration having been dealt a significant blow after not even a month in office, President Donald Trump now must fill a vital national security post after the resignation of embattled national security adviser Michael Flynn.

Flynn stepped down late Monday, ending days of speculation about his fate following reports that he had misled Vice President Mike Pence and other officials about his contacts with Russia.

Flynn conceded that discussions of sanctions may have come up during several calls with the Russian ambassador during the transition period leading up to Trump’s Jan. 20 inauguration.

He acknowledged that he gave “incomplete information” about those discussions to Pence, who, apparently relying on information from the national security adviser, initially said Flynn had not discussed sanctions with the Russian envoy.

Trump named retired Lt. Gen. Keith Kellogg as acting national security adviser and a senior administration official said he was one of three candidates the president was considering to replace Flynn on a permanent basis.

Whoever emerges as Trump’s choice will take the helm of the National Security Council at a time when the young administration is grappling with a series of national security challenges, including North Korea’s reported ballistic missile launch. The president, who was joined at his Mar-a-Lago estate in Florida by Japanese Prime Minister Shinzo Abe over the weekend, voiced solidarity with Japan.

The White House also is dealing with fallout from the rocky rollout of Trump’s executive order on immigration, which has been blocked by the courts. The order was intended to suspend the nation’s refugee program and bar citizens of seven Muslim-majority countries from entering the United States.

Here’s a look at the top named contenders for the post, which does not require Senate confirmation:

RETIERD LT. GEN. KEITH KELLOGG

Kellogg had previously been appointed as the National Security Council chief of staff and, along with Flynn, advised Trump on national security and foreign policy issues during the campaign. He had been considered for national security adviser before the post went to Flynn.

Kellogg was chief operating officer of the Coalition Provisional Authority in Iraq, the interim governing body following the fall of Saddam Hussein in 2003. He previously worked as executive vice president of research and technology for Virginia-based information technology firm CACI International, which works as a contractor for defense, intelligence and homeland security agencies.

DAVID PETRAEUS

The most audacious choice would likely be former CIA Director Petraeus. A retired four-star general, he was bounced from his position atop the intelligence agency in 2012 after he it was revealed that he passed on classified information to his biographer, who had also become his mistress.

But Trump during the campaign spoke sympathetically about Petraeus’ plight despite his frequent criticisms of his Democratic opponent, Hillary Clinton, for mishandling classified materials. Petraeus was briefly under consideration to become secretary of state before Trump picked ExxonMobil CEO Rex Tillerson.

RETIRED VICE ADM. ROBERT HARWARD

Harward, a Navy SEAL, served as deputy commander of the United States Central Command when it was under the command of Gen. James Mattis, who is now secretary of defense. He served on the National Security Council for President George W. Bush and commissioned the National Counter Terrorism Center.

Upon retirement in 2013 after a nearly 40-year career in the Navy, Harward took a post as a chief executive officer for defense at aerospace giant Lockheed Martin in the United Arab Emirates. Trump has recently been in very public negotiations with Lockheed over the cost of its F-35 fighter jet program.