Before the police made a move, Clarke wanted proof — fingerprints ideally — that they had the right guy. Miki found one on file from Tulja’s ID card application and sent it to the F.B.I.’s fingerprint database in West Virginia, where Dzurlic’s prints were stored from his stateside arrests. On Feb. 8, Clarke’s phone rang in the middle of the night. The prints lined up.

Since the extradition treaty between Montenegro and the United States does not apply to Montenegrin citizens, Clarke and his colleagues were forced to weigh several options. One was to lure Dzurlic out of Montenegro on a false premise and arrest him in a country from which he could be extradited. They settled instead on extraterritorial prosecution, where Montenegro would prosecute Dzurlic according to its own legal procedures and sentencing guidelines for the crimes he allegedly committed in New York. Just two weeks after Clarke briefed Miki, on a cold, windy night in February 2007, around two dozen Montenegrin policemen fanned out around the terra-cotta-topped wall that surrounded Dzurlic’s house. A team of plainclothes officers scaled the aluminum staircase leading to the entrance. One of the officers rapped on the door. No one answered. He knocked again.

Dzurlic came to the door wearing a green zip-up sweater and jeans. The officers spun him around and handcuffed him, then streamed into the apartment to search the premises. After an hour, they brought Dzurlic outside and put him into the back seat of a patrol car.

While the Montenegrin prosecutors leafed through stacks of old police files from New York, Clarke probed the unsolved murders in Albania and Belgium. He couldn’t get over the similarities between the murder of Mary Beal, the homicides in Belgium and the victim from Albania. Five months after Dzurlic’s arrest, Clarke flew to Belgium, along with Craig Ackley and Dan Bermingham from the B.A.U. They wanted to discuss the possibility that Dzurlic was the person the Belgium press had coined le dépeceur de Mons, or the Butcher of Mons. Between January 1996 and July 1997, five women were murdered and dismembered in the vicinity of Mons, a quaint city of about 100,000 people in southern Belgium. Ackley too had been examining the placement of the cuts and the instruments used, and, like Clarke, found the murders to be “very, very similar.”

Soon after the F.B.I. delegation arrived at the police station in Mons, Ackley broke out his laptop and briefed the Belgian authorities on all these commonalities. The Belgians, however, were not persuaded. “The party line was that Smajo Dzurlic was a ‘person of interest,’ ” Clarke told me. “They knew about Smajo and had a picture of Smajo. They said they had done everything in their power to try and put Smajo in Mons during the time frame of the murders but had come up empty.” According to cables acquired through the Freedom of Information Act, the Belgian police believed Dzurlic had fled the country as early as June 1995 — six months before the first murder occurred in Mons — when his son tipped him off that the Americans wanted him in connection with Beal’s death. When Dzurlic was finally in custody, Belgian officials were so certain that he wasn’t the Butcher of Mons that they showed no interest in questioning him about it. “The trail of Dzurlic has been explored,” Dominique Francq, a prosecutor in Mons, told me. “Was he in Belgium? Yes. But not in Mons. If that was the case, he would now be in prison, and I would be drinking Champagne.”