His excited breaths visible in front of him, Kirchner climbed his building’s ladder, leaped onto the roof, and hopped over to the roof next door. The roof hatch was secured by simple pins, which rattled off with only a little encouragement. He looked down and saw Spielbergian shafts of light slicing past the boarded-up windows. He was in.

He thought about going down, but then stopped. What if I fall, break my leg, starve to death, turn into a pile of bones—and whoever buys the building finds my bones? It was a rather transparent fantasy: Not only does he die, he doesn’t get the house.

He darted back to his apartment, called his friend Fabien Pisani, with whom he had dinner plans, and left a message: “If I don’t show up tonight,” he said, “call the authorities and send them to the building next door.” He grabbed a flashlight, a cell phone, and a steak knife—squatters? drug dealers?—and headed up, out, and back down the hatch.

What he saw was at once familiar and bizarre: The top floor had been gutted and was in the midst of being framed out to look a lot like his own apartment. The second floor was the same. It seemed like a construction site where the workers had just left for the day, never to return. And there was a lot of work left to be done. He saw ancient pine subflooring that was warped and bowed—and the ground floor was a disaster, completely unrenovated. Kicking up clouds of dust that got in the way of his flashlight, Kirchner saw a pile of envelopes in a heap beneath the front door’s mail slot. The mail seemed untouched.

Pauline and José apparently had been living on the ground floor while renovating upstairs. They did not live especially well. Clothes were packed away on makeshift racks. Kirchner kicked open the door of a side room and saw a tiny kitchen and living space with a daybed, its mattress so well worn that it still had a body-shaped imprint, and he could see the oily residue from someone’s head. There was a Pompeii aspect to the place, everything frozen in time.

Kirchner left through the hatch; he didn’t want anyone seeing him come out the front door. That night, he told Fabien what he did.

“You are a crazy man,” Fabien said in his thick Cuban accent. “But keep me posted.”

Kirchner went back practically every day after that. He found the keys in a little bowl on the kitchen table, and he worked up the nerve to go in and out through the front door. “In a way,” he says, “I now had possession of the building.”

A week or so later, Kirchner’s investigation hit a wall. His courthouse leads were running dry. He’d run out of neighbors to grill. The time seemed right to commit another crime. He gathered José and Pauline’s pile of mail in a large trash bag and went through everything. He found takeout menus, mostly; a warning letter from a city marshal; letters from agencies that monitor the foreclosure lists, offering cash for the building. “Competitors,” Kirchner says. Others were notes from neighbors making similar offers, often in broken English: I am not trying to steal your building. I want to offer you fair marketable price . . .

“It was ironic,” he says now. “I had this kind of disgust for that whole approach—sharks circling a wounded seal or something—but I’m on the inside, kind of snickering. These people were idiots—you’ve got to get in the building.”

In the kitchen garbage can, he found a real clue—a 1998 wall calendar that laid out the final six months of José and Pauline’s lives in New York. It had doctors’ appointments, physicians’ names and numbers, and information about a closing of another building the couple had sold on Vanderbilt Street, a few blocks away. “Which I knew about, because I had run their name through the deed department.” He called all the doctors, and after a certain amount of pestering—I’m a concerned neighbor, and I think something might have happened to them—one relented and told Kirchner that Pauline had indeed been ill, with breast cancer that had metastasized to her brain. As of 1998, her doctors believed she didn’t have long to live.

Behind the oven, Kirchner found a half-burned letter from a Madrid branch of the Spanish bank Banesto. It had an account number and Pauline’s second last name, García. The letter was a notice of a transfer into the account of $125,000, just a few months before José and Pauline disappeared. Kirchner figured that Pauline and José must have sold the building on Vanderbilt, made a profit, and camped out temporarily in the Fifth Avenue property. They marked time there waiting for the funds—and, according to the calendar, immigration papers. On September 3, 1998, the calendar read, “Flight 904 TWA to Spain.” After that, nothing.

Kirchner called Pauline’s bank, but the teller wouldn’t divulge the kind of details that Pauline’s doctor had. Finally, he got a break. Buried underneath a pile of newspapers in a back bedroom was an odd wooden knickknack shaped like a deformed rabbit. The base had prongs for holding keys. One of the ears had a thermometer. Mounted under a circular ring was a postcard with an aerial view of a little village nestled in a green valley. The deformed rabbit must be a souvenir. But from where?