Stephen J. Rapp, the U.S. ambassador at large for war crimes, is stepping down after five and a half years as the Obama administration’s point man for global prosecutions of the world’s most notorious war criminals.

Rapp’s departure has not yet been announced, and the war crimes prosecutor did not respond to requests for confirmation. But several sources familiar with Rapp’s plans confirmed that he is leaving and that the State Department has formally launched a search campaign for a replacement. It remains unclear, however, how soon he will go.

Rapp was tapped by Barack Obama’s administration in April 2009 to underscore its effort to champion the cause of the world’s most desperate victims. Rapp is part of a small group of American officials, including Samantha Power, the U.S. ambassador to the United Nations, who came to the administration with a reputation for championing human rights and holding those responsible for war crimes to account. But they have struggled to put those principles into practice in an administration that has often failed to live up to its own lofty rhetoric.

Rapp started out his job vowing to reassert Washington’s leadership on matters of international justice, which had been tarnished by the Bush administration’s excesses during the war on terror.

Human rights activists generally praised Rapp’s performance and said he was the administration’s most passionate advocate for deeper U.S. support for the International Criminal Court (ICC). His departure could remove a key American ally of the tribunal, based in The Hague, as it faces a politically controversial dispute over Palestine’s recent move to join the court as a way of bringing war crimes cases against Israel. Lawmakers have promised to slash American aid to the Palestinian Authority in response, and the ICC — never popular on Capitol Hill — could come under congressional attack if it opens legal proceedings against Israeli officials.

Still, Rapp’s actual impact on the administration’s human rights policies has been more limited than many of his allies had hoped. The administration has largely pressed the case for accountability for crimes selectively, passionately promoting the international prosecution of political rivals, from late Libyan leader Muammar al-Qaddafi to Syrian President Bashar al-Assad, while seeking to protect American military personnel and allies like Israel from international scrutiny for their own alleged crimes, according to rights advocates. The Obama administration has made it clear that it has no intention of formally joining the International Criminal Court, arguing that a push for Senate ratification of the treaty establishing the court would be destined for defeat.

The administration’s standing on such issues has also been tainted by its refusal to punish CIA operatives involved in the torture of detainees or to close down the U.S. detention facility at Guantánamo Bay, a source of widespread outrage around the world. Early this week, Sen. John McCain (R-Ariz.) and other key Republican leaders introduced legislation that would block the administration’s effort to close Guantánamo before the president completes his term.

“Stephen Rapp has been a welcome voice of integrity and determination in the pursuit of justice,” said James Goldston, a former U.N. trial attorney who serves as the executive director of the Open Society Justice Initiative. But he said Rapp’s tenure has coincided with a period in which the “United States as a whole has somewhat diminished credibility on the issue of accountability for grave crimes.”

“The most important thing the U.S. can do to restore its credibility to speak on issues of accountability are to close Guantánamo and prosecute those responsible for torture in the United States,” he said. “The mission is not accomplished, and it is very clear that as excellent as he has been, there is only so much he can do [to restore America’s standing].”

Rapp, 65, began his career as a victim: As a 21-year-old congressional intern in Washington, D.C., he was kidnapped by three men, pistol-whipped, stowed in the trunk of his 1961 Chevrolet, and taken on a crime spree that he thought might be his last ride. The experience set him on a career pursuing violent criminals, initially as a federal prosecutor in Iowa and later as a U.N. prosecutor for Rwanda and Sierra Leone, where he prosecuted mass murderers and alleged war criminals including former Liberian President Charles Taylor.

Rapp cultivated a reputation as a persistent advocate, traveling widely and frequently and visiting war crimes trials in far-off places, including Guatemala, Bangladesh, and Cambodia (he logged 222 days of travel in 2012 alone). He devoted much of his time and energy to helping to strengthen national courts’ capacity for trying their own war criminals.

Goldston and other advocates credit Rapp for seeking practical ways to advance the cause of accountability for mass atrocities on a range of fronts, exploring a host of options for prosecuting Syrian rights abusers outside Syria and enhancing American support for the International Criminal Court, which the United States has never joined.

Rapp has championed administration efforts to provide political and diplomatic support for the ICC, including paving the way for the payment of financial rewards for information leading to the arrests of fugitives wanted by the ICC. U.S. Special Forces in the Central African Republic recently detained Dominic Ongwen, a senior commander of the Lord’s Resistance Army who had a $5 million bounty on his head. Ongwen will be handed over to authorities in the Central African Republic and surrendered to the ICC to face charges before the court, Uganda’s military spokesman, Paddy Ankunda, said on Twitter.

“I think Stephen Rapp is in the front ranks of those in the administration supporting the court in a more robust way,” said Richard Dicker, a supporter of the court at Human Rights Watch. “He is going to leave some very large shoes to be filled.”

But Dicker said that Washington’s commitment to the application of justice is “situational,” providing support on the one hand for investigations in Darfur and the Central African Republic, but withholding it in others. For instance, the United States voted in favor of a U.N. Security Council resolution authorizing an ICC investigation into alleged crimes by the Qaddafi regime. But U.S. support for an investigation waned after a pro-American government seized power. “After the Qaddafi regime was overthrown the Obama administration went silent,” Dicker said.

In Syria, where as many as 200,000 civilians have died, American efforts to punish the Assad regime for its widespread human rights abuses have gone nowhere. The United States agreed in May to back a French push for a resolution authorizing an investigation into mass crimes in Syria by the International Criminal Court. But the measure was vetoed by Russia and China. Following the veto, Rapp approached a number of governments, including those of Jordan and Turkey, to promote the idea of a regional court. But Rapp later conceded it had gained little traction, including within the U.S. government. “This idea [of a regional war crimes court] has not yielded a significant quantum of support that would overcome the kind of legal and precedential aspects,” Rapp told Foreign Policy in an interview in November.

Undeterred, Rapp tried another tack, lobbying governments whose nationals have been victimized in Syria to consider prosecuting their abusers back home in their own national courts, invoking a legal principle known as “universal jurisdiction.” In the interview, Rapp said that FBI officials scouring a trove of more than 50,000 photos smuggled out of the country by a defector, known as Caesar, have identified a handful of foreign victims. “Some of the photos [from the Caesar report] are non-Syrians, and we are prepared to share that material with countries who might be prepared to prosecute [suspected perpetrators] in national courts,” Rapp said at the time.

Bill Wiley, the founder of the Commission for International Justice and Accountability, which sends teams into Syrian war zones to collect court-ready evidence of war crimes, said that Rapp had been critical in supporting his group’s efforts to collect evidence of war crimes in Syria.

“I can’t sing loudly enough in praise of Stephen. He was extremely helpful with the Europeans,” Wiley said in an interview. “We would line up the funding and do the paperwork and shop the idea. But we weren’t known from Adam. He’d pick up the phone and call and say, ‘This is who I am, and I will vouch for these guys. What they’re doing is important.'”

Former U.S. Secretary of State Madeleine Albright formed the war crimes office in 1997 to lend U.S. support to U.N.-backed war crimes courts in Yugoslavia and Rwanda. The office’s mission changed during George W. Bush’s administration, when it served a role as defender of U.S. counterterrorism policy while approaching countries willing to take in detainees held at Guantánamo Bay.

“It was a radical reversal,” David J. Scheffer, the first U.S. ambassador at large for war crimes, said in an interview in 2009. It changed the office’s mission from “one of going out there and advancing the agenda of international justice to one of trying to shield the U.S. from justice inquiries,” he said.

Today, the war crimes office no longer deals with Guantánamo Bay. It helps war-racked countries going through political transitions set up truth commissions or build the capacity of their courts to prosecute war crimes. Rapp has made numerous trips to Bangladesh to urge authorities to include fairness provisions in the special courts set up to try victims of mass crimes during the country’s war of liberation in 1971. He has also lent support to criminal courts in Lebanon, Guatemala, and Cambodia, where a U.N.-backed Cambodian court is prosecuting leaders of the former Khmer Rouge government. In Sri Lanka, Rapp has conceded that it is unrealistic to expect that Colombo will assent to conduct investigations into mass war crimes by government forces during the bloody final months of the country’s civil war. But he has urged the government to at least exhume the bodies of victims in order to give their families some sense of closure.

Steven Ratner, a law professor at the University of Michigan and a former member of a U.N. panel that examined the commission of mass atrocities in the final months of the Sri Lankan war, said Rapp has effectively raised the profile of the post and improved U.S. relations with the ICC. But he said Rapp was only one player in the administration’s push to confront mass atrocities.

The establishment of the Atrocities Prevention Board, the brainchild of Power, the U.S. ambassador to the United Nations, had a far-reaching impact because it integrated the consideration of mass murder into the administration’s decision-making process.

Since taking office, Obama has authorized military force to protect civilians from Libya in 2011 to Iraq in 2014. The United States has also deployed force to aid Ugandan troops in the hunt for Joseph Kony, the notorious leader of the Lord’s Resistance Army. But the impact has been mixed. The NATO-led military intervention in Libya has left that country in chaos. The United States has little to show for its training of Nigerian troops to confront the extremist Islamist organization Boko Haram.

Ratner said Rapp had little success forcing American intelligence agencies to share information they’ve collected about human rights abuses abroad. Ratner recalled paying a visit to Rapp’s office while Ratner was investigating war crimes in Sri Lanka on behalf of the U.N. panel, which was set up by U.N. Secretary-General Ban Ki-moon. Ratner was seeking information about allegations that Sri Lankan officials summarily executed senior rebel commanders after the United Nations brokered their surrender. Ratner had heard a rumor that the U.S. Embassy had picked up some intercepts with information related to the case.

“They made it clear to us there were pretty strong constraints on what they were going to be able to share with us,” Ratner said. In the end, he said, “they