US President Donald Trump has announced the departure of Mr John Bolton as his national security adviser, the third person to hold the job since the beginning of the Trump administration.
Mr Trump has said he will announce a successor next week.
A shortlist of possible replacements has emerged.
Mr Charles Kupperman, a former Reagan administration official and defence contracting executive, is a long-time Bolton associate. Mr Kupperman, 68, was appointed in January as deputy national security adviser.
The President has appreciated Mr Kupperman's just-the-facts style compared with Mr John Bolton's often ideologically charged delivery: If Mr Donald Trump had to have a national security brief concerning long-term planning, he preferred it from Mr Kupperman as opposed to Mr Bolton, according to a person with knowledge of that process.
Mr Stephen Biegun, US Special Representative for North Korea, is considered a capable technocrat rather than a big-ideas person, unlike Mr Bolton, who had firm ideological views that shaped his policy positions. Recently, Mr Biegun has been in closer alignment with Secretary of State Mike Pompeo and Mr Trump than with the hardline, anti-North Korea views of Mr Bolton.
Mr Biegun, 56, also served as an executive secretary of the National Security Council under former president George W. Bush. In August 2001, Mr Biegun was with the president, then on vacation at his ranch in Texas, when Mr Bush received a daily brief containing an article titled "Bin Laden determined to strike in US".
Mr Brian Hook, 51, is Special Representative for Iran and a senior adviser to Mr Pompeo. A lawyer brought into the State Department under Mr Rex Tillerson, he is one of the remaining survivors from that era.
An administration official familiar with Mr Hook's relationship with Mr Trump said that the two "interact on Iran" and that "the President is happy with how the strategy is going there".
He would also probably have the support of the President's son-in-law and senior adviser Jared Kushner, who has tried to push his allies into high-profile administration positions before.