Hargrove estimates that two thousand serial killers are at large in the U.S. Illustration by Harry Campbell Audio: Listen to this story. To hear more feature stories, download the Audm app for your iPhone.

Thomas Hargrove is a homicide archivist. For the past seven years, he has been collecting municipal records of murders, and he now has the largest catalogue of killings in the country—751,785 murders carried out since 1976, which is roughly twenty-seven thousand more than appear in F.B.I. files. States are supposed to report murders to the Department of Justice, but some report inaccurately, or fail to report altogether, and Hargrove has sued some of these states to obtain their records. Using computer code he wrote, he searches his archive for statistical anomalies among the more ordinary murders resulting from lovers’ triangles, gang fights, robberies, or brawls. Each year, about five thousand people kill someone and don’t get caught, and a percentage of these men and women have undoubtedly killed more than once. Hargrove intends to find them with his code, which he sometimes calls a serial-killer detector.

Hargrove created the code, which operates as a simple algorithm, in 2010, when he was a reporter for the now defunct Scripps Howard news service. The algorithm forms the basis of the Murder Accountability Project (MAP), a nonprofit that consists of Hargrove—who is retired—a database, a Web site, and a board of nine members, who include former detectives, homicide scholars, and a forensic psychiatrist. By a process of data aggregating, the algorithm gathers killings that are related by method, place, and time, and by the victim’s sex. It also considers whether the rate of unsolved murders in a city is notable, since an uncaught serial killer upends a police department’s percentages. Statistically, a town with a serial killer in its midst looks lawless.

In August of 2010, Hargrove noticed a pattern of murders in Lake County, Indiana, which includes the city of Gary. Between 1980 and 2008, fifteen women had been strangled. Many of the bodies had been found in vacant houses. Hargrove wrote to the Gary police, describing the murders and including a spreadsheet of their circumstances. “Could these cases reflect the activity of one or more serial killers in your area?” he asked.

The police department rebuffed him; a lieutenant replied that there were no unsolved serial killings in Gary. (The Department of Justice advises police departments to tell citizens when a serial killer is at large, but some places keep the information secret.) Hargrove was indignant. “I left messages for months,” he said. “I sent registered letters to the chief of police and the mayor.” Eventually, he heard from a deputy coroner, who had also started to suspect that there was a serial killer in Gary. She had tried to speak with the police, but they had refused her. After reviewing Hargrove’s cases, she added three more victims to his list.

Four years later, the police in Hammond, a town next to Gary, got a call about a disturbance at a Motel 6, where they found a dead woman in a bathtub. Her name was Afrikka Hardy, and she was nineteen years old. “They make an arrest of a guy named Darren Vann, and, as so often happens in these cases, he says, ‘You got me,’ ” Hargrove said. “Over several days, he takes police to abandoned buildings where they recover the bodies of six women, all of them strangled, just like the pattern we were seeing in the algorithm.” Vann had killed his first woman in the early nineties. In 2009, he went to jail for rape, and the killings stopped. When he got out, in 2013, Hargrove said, “he picked up where he’d left off.”

Researchers study serial killers as if they were specimens of natural history. One of the most comprehensive catalogues is the Radford Serial Killer Data Base, which has nearly five thousand entries from around the world—the bulk of them from the United States—and was started twenty-five years ago by Michael Aamodt, a professor emeritus at Radford University, in Virginia. According to the database, American serial killers are ten times more likely to be male than female. Ray Copeland, who was seventy-five when he was arrested, killed at least five drifters on his farm in Missouri late in the last century, and is the oldest serial killer in the database. The youngest is Robert Dale Segee, who grew up in Portland, Maine, and, in 1938, at the age of eight, is thought to have killed a girl with a rock. Segee’s father often punished him by holding his fingers over a candle flame, and Segee became an arsonist. After starting a fire, he sometimes saw visions of a crimson man with fangs and claws, and flames coming out of his head. In June of 1944, when Segee was fourteen, he got a job with the Ringling Brothers circus. The next month, the circus tent caught fire, and a hundred and sixty-eight people were killed. In 1950, after being arrested for a different fire, Segee confessed to setting the tent ablaze, but years later he withdrew his confession, saying that he had been mad when he made it.

Serial killers are not usually particularly bright, having an average I.Q. of 94.5, according to the database. They divide into types. Those who feel bound to rid the world of people they regard as immoral or undesirable—such as drug addicts, immigrants, or promiscuous women—are called missionaries. Black widows kill men, usually to inherit money or to claim insurance; bluebeards kill women, either for money or as an assertion of power. A nurse who kills patients is called an angel of death. A troller meets a victim by chance, and a trapper either observes his victims or works at a place, such as a hospital, where his victims come to him.

The F.B.I. believes that less than one per cent of the killings each year are carried out by serial killers, but Hargrove thinks that the percentage is higher, and that there are probably around two thousand serial killers at large in the U.S. “How do I know?” he said. “A few years ago, I got some people at the F.B.I. to run the question of how many murders in their records are unsolved but have been linked through DNA.” The answer was about fourteen hundred, slightly more than two per cent of the murders in the files they consulted. “Those are just the cases they were able to lock down with DNA,” Hargrove said. “And killers don’t always leave DNA—it’s a gift when you get it. So two per cent is a floor, not a ceiling.”

Hargrove is sixty-one. He is tall and slender, with a white beard and a skeptical regard. He lives with his wife and son in Alexandria, Virginia, and walks eight miles a day, to Mount Vernon or along the Potomac, while listening to recordings of books—usually mystery novels. He was born in Manhattan, but his parents moved to Yorktown, in Westchester County, when he was a boy. “I lived near Riverside Drive until I was four,” he said. “Then one day I showed my mom what I learned on the playground, which is that you can make a switchblade out of Popsicle sticks, and next thing I knew I was living in Yorktown.”

Hargrove’s father wrote technical manuals on how to use mechanical calculators, and when Hargrove went to college, at the University of Missouri, he studied computational journalism and public opinion. He learned practices such as random-digit-dialling theory, which is used to conduct polls, and he was influenced by “Precision Journalism,” a book by Philip Meyer that encourages journalists to learn survey methods from social science. After graduating, in 1977, he was hired by the Birmingham Post-Herald, in Alabama, with the understanding that he would conduct polls and do whatever else the paper needed. As it turned out, the paper needed a crime reporter. In 1978, Hargrove saw his first man die, the owner of a convenience store who had been shot during a robbery. He reported on a riot that began after police officers shot a sixteen-year-old African-American girl. Once, arriving at a standoff, he was shot at with a rifle by a drunk on a water tower. The bullet hit the gravel near his feet and made a sound that “was not quite a plink.” He also covered the execution of a man named John Lewis Evans, the first inmate put to death in Alabama after a Supreme Court abrogation of capital punishment in the nineteen-sixties and seventies. “They electrocuted people in Alabama in an electric chair called the Yellow Mama, because it was painted bright yellow,” Hargrove said. “Enough time had passed since the last execution that no one remembered how to do it. The first time, too much current went through too small a conduit, so everything caught fire. Everyone was crying, and I had trouble sleeping for days after.”