President Trump announced Wednesday he has chosen current Special Presidential Envoy for Hostage Affairs Robert C. O’Brien as national security adviser, replacing Ambassador John Bolton.

Trump revealed his pick on Twitter: “I am pleased to announce that I will name Robert C. O’Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job!”

I am pleased to announce that I will name Robert C. O’Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job! — Donald J. Trump (@realDonaldTrump) September 18, 2019

O’Brien helped to gain the release of American rapper A$AP Rocky, after he was arrested and detained in Sweden earlier this year after getting into a street altercation.

He served as a senior foreign policy advisor to then-Gov. Mitt Romney (R-MA) and Gov. Scott Walker (R-WI) in their presidential campaigns. Romney tweeted Wednesday, after the president made the announcement of his appointment:

Congratulations to my friend Robert O’Brien, who the President has appointed to serve as National Security Advisor. As the Special Presidential Envoy for Hostage Affairs, he has doggedly pursued the release of American hostages abroad. He is a man of the highest integrity. — Senator Mitt Romney (@SenatorRomney) September 18, 2019

O’Brien has some experience with Afghanistan, which the president is trying to wind down the U.S. troop presence in. He served as as Co-Chairman of the U.S. Department of State Public-Private Partnership for Justice Reform in Afghanistan under the Bush and Obama administrations, according to his State Department biography.

He also served as a member of the U.S. Cultural Property Advisory Committee, which advises the government on issues relating to the trafficking of antiquities and other cultural items. In 2005, he served as a U.S. Representative to the 60th session of the United Nations General Assembly, where he worked with then-U.S. Ambassador to the UN John Bolton. He also served as a major in the U.S. Army Reserve.

O’Brien is also the co-founding partner of litigation firm Larson O’Brien LLP in Los Angeles, which focuses on complex litigation and international arbitration. He is a graduate of the Boalt Hall School of Law at U.C. Berkeley and UCLA.

He replaces Bolton, who the president fired last week, over various foreign policy disagreements reportedly on North Korea, Iran, Venezuela and Afghanistan.

This is a developing story.