After the conflict in Kosovo ended, in June, 1999, a tribunal in The Hague set out to punish the perpetrators of atrocities. Louise Arbour, the lead prosecutor, described Kosovo—a former province of the Federal Republic of Yugoslavia—as “one vast crime scene.” Her investigators’ primary target was Slobodan Milosevic, the Serb who led Yugoslavia. Milosevic had claimed that Kosovo’s majority group, ethnic Albanians, treated ethnic Serbs like slaves, and armed forces under his control had waged a campaign of mass murder there, slaughtering more than ten thousand Kosovars. NATO, relying heavily on American forces, launched air strikes to stop Milosevic, and received crucial assistance from the Kosovo Liberation Army, the main rebel force in the region. K.L.A. scouts relayed Serbian tank coördinates to Green Berets, who passed on the information to American fighter jets. Three months after the air strikes began, Serbia surrendered. Milosevic was eventually arrested and sent to The Hague, where he died in prison, of a heart attack.

In Kosovo, meanwhile, the K.L.A. officially disbanded, but many of its members joined political parties. A party led by Hashim Thaci, the K.L.A.’s political chief, rose to power. In 2008, Thaci became the first Prime Minister of an independent Kosovo. Two years later, Vice-President Joe Biden hailed him as “the George Washington of Kosovo.”

Today, nearly two thousand people remain missing in Kosovo, a country that José Pablo Baraybar, a Peruvian who headed the U.N.’s Office on Missing Persons and Forensics, described to me as “one of the most exhumed places on earth.” DNA technology has helped investigators identify hundreds of bodies, many of them buried in mass graves. But, even in an era of sophisticated forensic science, definitive evidence can be elusive: in one case, the Serbs burned hundreds of corpses in a lead smelter.

Dozens of Serbs have been convicted of war crimes since the fighting stopped, but they were not the only ones responsible for violence. In Pristina, the capital of Kosovo, rumors circulated that, in the summer of 1999, K.L.A. paramilitaries had trucked prisoners across the border to secret detention camps in Albania, where they were tortured and, sometimes, killed. But, in public, Kosovars embraced a “silence taboo,” in the words of Vehbi Kajtazi, a journalist in Pristina. The men who led the K.L.A. remained a fearsome presence in the country, posing a threat to anyone who spoke out, and ethnic Serbs were a powerless bloc, falling to less than two per cent of the population. Unlike Argentina, South Africa, and Sierra Leone, Kosovo failed to establish its own truth-and-reconciliation commission.

The task of accounting for the missing was left largely to outsiders. One of them was Michael Montgomery, an American radio journalist who had helped expose the massacre of forty-one Kosovar Albanians by Serbian forces in the village of Qyshk, on May 14, 1999. He began amassing troubling stories involving the K.L.A. Multiple sources told him that, in the days after Milosevic’s defeat, the K.L.A. had shipped accused traitors to camps in Albania. A former K.L.A. member recalled guarding seven prisoners in the back of a van, their mouths taped and their hands cuffed, as they crossed the border. A K.L.A. driver said that he had been given orders not to hurt anyone; once his captives were in Albania, they were taken to a house where doctors were present. The driver heard that the doctors sampled the prisoners’ blood and assessed their health. Several sources implied that this caretaking had a sinister purpose: the K.L.A. was harvesting the prisoners’ organs and selling them on the black market.

Montgomery was concerned that these stories might be propaganda planted by the Serbian government, so he tracked down additional sources. Three people recalled taking prisoners to a yellow house outside the Albanian town of Burrel. Another K.L.A. driver told Montgomery that there were only two places where he “brought people but never picked anyone up”: the yellow house and a cream-colored farmhouse near the airport in Tirana, Albania’s capital. The farmhouse, he noted, had a “very strong smell of medicine.” The driver added that he sometimes heard other drivers talking about “organs, kidneys, and trips from the house to the airport.” Since the late nineteen-nineties, Istanbul—a short flight from Tirana—has been a destination for transplant tourism.

In late 2002, a K.L.A. member told Montgomery that the group had made “a fortune” by trafficking body parts, primarily kidneys. C., as Montgomery called the source, claimed that the K.L.A. received about forty-five thousand dollars per body. Most shipments involved body parts from “two or three Serbs,” though C. knew of an instance when the K.L.A. “did five Serbs together.”

In late 2002 and early 2003, Montgomery travelled with a colleague to Albania, carrying a map drawn by his informants. It directed them to the yellow house and to the farmhouse near the airport. But they didn’t knock on the doors. Montgomery thought that they needed stronger evidence before confronting the occupants. “The only way we felt we could report this was if bodies were recovered and matched with missing people,” he told me.

Montgomery decided to put his investigations aside, but he didn’t let the matter go entirely. He sent a memo to the U.N.’s missing-persons office in Kosovo, asserting that, in 1999 and 2000, between one hundred and three hundred prisoners were taken to Albania, where some were dispatched to a “makeshift clinic” that extracted “body organs from the captives.” The U.N. forwarded the memo to the International Criminal Tribunal for the former Yugoslavia, or I.C.T.Y. The tribunal, established in The Hague in 1993, was designed to bring a measure of justice to those who had suffered horrors in the Balkans. The Serbs did not see the tribunal as impartial, and they have remained hostile to it. Tomislav Nikolic, Serbia’s current President, said recently that the tribunal “was founded to try the Serbian people.” In fact, most of the convictions in I.C.T.Y. courtrooms have been of ethnic Serbs, in part because the Milosevic regime made little effort to conceal its crimes. In Qyshk, Serbian militia members responsible for the massacre left behind photographs of themselves posing with machine guns; the images were later used to identify some of the culprits.

The lead prosecutor at the I.C.T.Y. was Carla Del Ponte, an indefatigable fifty-six-year-old lawyer from the Italian-speaking region of Switzerland. She had joined the tribunal in September, 1999, and she had tried dozens of Serbs for crimes against humanity. The K.L.A., she suspected, had committed significantly fewer war crimes, but scale is not exculpatory.

Del Ponte received Montgomery’s memo as she was preparing to indict Ramush Haradinaj, a former bouncer who joined the K.L.A., rose to commander, and then went on to become the Prime Minister of U.N.-administered Kosovo. She intended to charge Haradinaj with “planning, instigating, ordering, committing, or otherwise aiding and abetting” the abuse of dozens of prisoners. The indictment also accused him of participating in “the abduction of persons who were later found murdered.” In documents sent to Del Ponte, Montgomery quoted his source C. saying that Haradinaj “must have known” about organ trafficking, in part because his brother Daut was close to someone “heavily involved” in the trade. (In 2002, a U.N. court convicted Daut, a former K.L.A. member, of involvement in the murder of four political rivals.)