WASHINGTON (AP) — President Donald Trump said Wednesday he's naming hostage negotiator Robert O'Brien, who Trump had monitor the criminal case in Sweden against American rapper A$AP Rocky, as his new national security adviser.

Trump tweeted the announcement from California, where he was opening the third and final day of a West Coast fundraising swing.

"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," Trump said. "I have worked long & hard with Robert. He will do a great job!"

Trump announced that O'Brien will become his fourth national security adviser a week after he ousted John Bolton , citing policy disagreements. O'Brien was among five candidates the Republican president had said Tuesday were being considered for the post, which does not require Senate confirmation.

As the special presidential envoy for hostage affairs at the State Department, O'Brien worked closely with the families of American hostages and advised administration officials on hostage issues. He previously helped lead the department's public-private partnership for justice reform in Afghanistan during the Bush and Obama administrations.

Earlier this year, Trump dispatched O'Brien to Sweden to keep tabs on the criminal case there against A$AP Rocky, who a judge and jury ultimately found guilty of assault in August, several weeks after a street brawl in Stockholm that attracted Trump's attention.

Trump tried to intervene in the case by calling Swedish Prime Minister Stefan Lofven in July and offering to personally guarantee bail for the rapper, whose real name is Rakim Mayers. Lofven told Trump he couldn't interfere in a legal matter.

From 2008 through 2011, O'Brien was a presidentially appointed member of a government committee that advises on issues related to the trafficking of antiquities and other cultural items. In 2005, President George W. Bush nominated O'Brien to be U.S. Representative to the U.N. General Assembly, where he worked with Bolton. O'Brien was confirmed by the Senate.

He also was an adviser on the Republican presidential campaigns of former Gov. Scott Walker of Wisconsin, former Gov. Mitt Romney of Massachusetts and Sen. Ted Cruz of Texas.

Earlier in his career, O'Brien was a senior legal officer for the U.N. Security Council commission that decided claims against Iraq that arose from the Gulf War. He was a major in the U.S. Army Reserve.

O'Brien has a law degree from the University of California-Berkeley and co-founded a law firm in Los Angeles focused on litigation and international arbitration issues. O'Brien is the author of "While America Slept," a collection of essays on U.S. national security and foreign policy billed as a "wake-up call to the American people." The book warned that the world had become more dangerous "under President Obama's lead-from-behind foreign policy."

Trump abruptly forced out Bolton on Sept. 10, after he and his hawkish national security adviser found themselves in strong disagreement over the administration's approach to Iran, Afghanistan and a host of other global challenges. The sudden exit marked the latest departure of a prominent voice of dissent from Trump's inner circle as the president has grown more comfortable following his gut instinct over the studious guidance offered by his advisers.

Bolton claimed he resigned and was not dismissed. Trump is scheduled to participate in the United Nations General Assembly next week.