US President Donald Trump entered his next phase of vetting for a new national security adviser, interviewing his top hostage negotiator Robert O’Brien at the White House on Friday, and combing through a growing list of candidates with diplomatic, military and business experience, The Washington Post reported.

The ouster of his hawkish national security adviser, John Bolton, has set off a turbulent contest at the Republican Party to install an adviser who will help guide the president as he seeks to broker a nuclear deal with North Korea, a withdrawal from Afghanistan, and an easing of tensions with Iran.

Secretary of State Mike Pompeo is quietly supporting three of his close colleagues, according to US officials and congressional aides familiar with the matter. Those candidates include O’Brien; Brian Hook, special envoy for Iran; and Ricky Waddell, assistant to the chairman of the Joint Chiefs of Staff.

Vice President Pence’s national security adviser, Keith Kellogg, and the president’s ambassador to Germany, Richard Grenell, are also under consideration for the position, the officials said.