A few years ago, Mike MacQueen, a historian working for the Department of Homeland Security, was at his desk combing through decades-old Bosnian military records, in search of war criminals who had eluded justice. The documents listed the names of top officers in a batallion implicated in the massacre of eight hundred Muslim prisoners at a schoolhouse and dam in eastern Bosnia, in 1995. He noticed that the name of one Bosnian Serb officer kept showing up in the logs: Ilija Josipović.

MacQueen had turned himself into an unlikely expert on the war that unfolded in the Balkans two decades ago, mastering the Serbo-Croatian language, making two dozen trips to the region, and becoming so well schooled in the war crimes that Bosnian prosecutors had flown him over repeatedly to testify at trials. He had familiarized himself with the names of many of the key figures involved in the atrocities, but he had never come across Josipović (pronounced yoh-SIP-oh-vitch). He made a note to himself to find out what happened to the Serb officer.

MacQueen, who is sixty-eight, has spent the last three decades tracking down war criminals who have been hiding in the United States. His role, first with the Justice Department and then with Immigration and Customs Enforcement, has been to find offenders who made it into America posing as refugees. His official title is senior historian, but MacQueen’s job description is more akin to that of a police detective.

Mike MacQueen in his office in Washington, in February, 2015. He works for the Human Rights Violators and War Crimes Unit, part of the Department of Homeland Security. Photograph by Drew Angerer / The New York Times / Redux

His obsession with war crimes has taken him overseas to interview survivors or obtain documents from authorities in Bosnia, Croatia, and Lithuania. Sometimes, it has meant knocking on the doors of unwitting suspects in the United States. But on many days, it has meant simply sitting in his office, not far from the Capitol, and examining one document after another from some three hundred thousand pages of records about the conflict that he has gathered.

It can be tedious work, MacQueen told me. A tiny phonetic mistake in a foreign dialect can imperil a case. MacQueen’s other preoccupation is building race cars. He uses the same detached tone to describe how he pieces together war-crimes cases as he does when explaining how he rebuilt an engine that blew out on his MG Midget during a recent race. Acts of mass killing can sound almost mundane as he recounts zeroing in on a suspected war criminal. “I guess it’s the banality of investigating evil,” he told me, a variation of Hannah Arendt’s famous phrase.

After finding Josipović’s name in the logs that day, MacQueen set out to learn what Josipović did during the war. Records listed him as an officer in a unit implicated in the Bosnian-Serb Army's murder of seven thousand Muslim men and boys at Srebrenica, in what would become Europe’s first genocide since the Second World War. He didn’t appear to be a low-level triggerman but, rather, an officer who rose in rank and gained responsibility over the course of the war. Where Josipović lived now was unclear. MacQueen knew that someone with his record should not have been able to get into the United States. But he also believed that hundreds of wartime offenders had posed as refugees amid an influx of about a hundred and twenty thousand Bosnians fleeing the conflict. As a precaution, MacQueen searched for Josipović’s name in ICE’s databases. A hit soon came back—from Akron, Ohio. “Josipović had fallen through the cracks,” MacQueen told me. He realized that one of the highest-ranking Bosnian war-crimes suspects he had ever identified had been living quietly in the United States since 2003.

Federal agents with “ICE” emblazoned on their jackets, conducting workplace raids and taking undocumented immigrants into custody, have become a notorious sight under President Trump. But investigators at a separate ICE unit, where MacQueen works, who are largely removed from the raging immigration debate, have carried out a much less visible mission during the past nine years, targeting human-rights offenders who came to America from dozens of countries.

The immigration group, officially known as the Human Rights Violators and War Crimes Unit, currently has more than a hundred and thirty-five active investigations of suspected foreign offenders now thought to be living in the United States, officials said. In April, the unit’s investigation into a Liberian warlord living outside of Philadelphia, who had been implicated in murders, rapes, and enslavement in his native country in the nineties, resulted in a thirty-year prison sentence for immigration fraud and perjury—the longest criminal sentence in the team’s history.

Most of MacQueen’s cases have involved Bosnian Serbs, the group blamed for the bulk of the war crimes during the conflict, but his unit has also moved to jail or deport a number of Bosnian Croats and Muslims who were also accused of atrocities. He says more than fifty of the Bosnian immigrants he investigated have been forced out of the country. (Many cases have not become public, because they came in sealed immigration proceedings.) In January, after thirty years in the federal government, MacQueen officially retired, but ICE asked him to stay on for another five years as a private contractor, because his work on the Balkan front was “invaluable,” Lisa Koven, the chief of ICE’s human-rights law section, said.

MacQueen is determined to continue working on the investigations of the suspects he has already identified, and help prosecutors prepare to take them to court. His aim is to finish what he began and get as many war criminals as he can forced out of the country. “I don’t really need the money,” he said.

A native New Yorker, MacQueen comes across as soft-spoken and stoic, with a wry sense of humor, but he admits to losing his temper with suspects whom he believes are lying to him. In an angry confrontation a few years ago with a Bosnian woman in Wisconsin who concealed her involvement with a Serbian military unit, he used a vulgar sexual expression in Serbo-Croatian to show what he thought of her claims of innocence. A judge “gave me a little talking to,” MacQueen said. “I have a stunning lack of sympathy for anyone with an unclean record. They can go fuck themselves.”

I first interviewed MacQueen five years ago, about his earlier work in hunting Nazis. I was writing a book about the thousands of Nazi war criminals who came to America after the Second World War, often with the assistance of American intelligence officials who saw them as potential spies against the Soviet Union. A source mentioned MacQueen’s role in breaking a critical case, in 1994, when he was at the Justice Department. For years, prosecutors suspected that a Lithuanian immigrant and naturalized American citizen in Massachusetts named Aleksandras Lileikis, who had led a special police force in Vilnius during the war, was a top Nazi collaborator who ordered the roundup of Lithuanian Jews in the nineteen-forties and turned them over to the Nazis for execution. But the Justice Department couldn’t prove it, and Lileikis denied any role in the massacres. “Show me something that I signed,” Lileikis had dared a prosecutor who showed up at his door, in Boston, in 1983.