On April 21, 2011, at an elegant three-story town house in the city of Nantes at the western end of the Loire Valley, French police discovered five bodies, all of them members of the notable Dupont de Ligonnès family, including the mother, Agnès, and her four children, Benoît, Anne, Thomas, and Arthur, ages 13 to 20. Each had been shot dead and each had been buried in the garden, along with the family’s two Labrador dogs. The sight stunned police.

Yet it wasn’t until they had fully examined the crime scene that investigators realized something was missing: namely the body of Xavier Dupont de Ligonnès, the father of the family and the sole male heir to one of France’s oldest familles de noblesse. In that moment he became one of France’s most wanted fugitives.

Father, husband, and loner: de Ligonnès seems to have developed two identities, a pious schoolboy who attended Mass each morning, and an aristocratic playboy who could be seen speeding about Versailles in his Triumph Spitfire.

Dupont de Ligonnès isn’t the first man from a proper French family to kill his wife and children. In 1993, Jean-Claude Romand, who had pretended for years to be a doctor with an important job at the World Health Organization, murdered his wife and children, his parents and their dog. The case was so confounding that Emmanuel Carrère wrote a book about it. (Romand was released from prison earlier this year after serving 26 years.)

The Dupont de Ligonnès affair involves a similarly horrible act as well as a member of France’s most privileged class, and so it has captivated the country, much like the Lord Lucan case once mesmerized Britain in the 1970s and 1980s. Lucan, for those who may not remember, vanished without a trace in 1974 after apparently killing the family’s nanny—whom he may have mistaken for his wife.

The Dupont de Ligonnès affair involves a similarly horrible act by a member of France’s most privileged class.

And like Lucan, the now vanished Dupont de Ligonnès has become a meme for mystery, as well as the inspiration for dinner-party jokes and Halloween costumes. His motives have spawned talk of Eyes Wide Shut–esque cultish rituals, while speculation as to his whereabouts ranges from Argentina to bolt-holes in far-flung French monasteries. There is even a theory that he is in the F.B.I.’s witness-protection program.

It’s a mystery that consumes France. This fall, high-level sources in France told reporters that Interpol agents received a tip that Dupont de Ligonnès would board a flight in Paris, bound for Glasgow. Yet when Scottish police arrested the suspect upon his arrival in Scotland, a DNA and fingerprinting test proved the man wasn’t Dupont de Ligonnès at all, but a retiree residing in the Parisian suburbs named Guy Joao. The false alarm not only gave French law enforcement a black eye; it ignited a second wave of talk-show jokes, social-media posts, and hashtag tweets, including one with Dupont de Ligonnès dressed up as Where’s Waldo with the caption “Where is Xavier Dupont de Ligonnès?”

But the dark humor surrounding his disappearance masks the grip the family’s murders have on France, a country that despite the Revolution still holds its aristocratic families in reverence and is rattled when financial ruin, sordid affairs, and inter-family violence destroy the lives of the Happy Few.

The Family

Until the murders, the Dupont de Mars de Ligonnès family (they dropped the “Mars” some time ago) wasn’t the kind of French family that spawned tawdry headlines. Theirs was a long line of noblesse oblige service in the military and the clergy, dating back to the Middle Ages. In 1754, Louis XV personally named a Dupont de Ligonnès a marquis, and an ancestor married the sister of the famed poet Lamartine. Their son Charles became the bishop of Rodez in the South of France, where he built a diocese on the street named after him.

In the village of Chanac, about 55 miles east of Rodez, the 16th-century château once held by the Dupont de Ligonnèses (Château de Ressouches) sits amidst 200-year-old chestnut trees. It’s considered a historic monument and open to the public on holidays dedicated to French heritage. People who visit the property may enter a chapel that displays the family coat of arms (a knight’s-armor visor accompanied by three silver stars). There is also a family tomb nearby, which holds generations of ancestors including Xavier’s late father, Bernard-Hubert. It was Bernard who moved his family north toward Paris and started what was known as the “Versailles branch.”

Although it’s part of the Greater Paris métropole, Versailles has always had its own eco-system. Today, most of the people who live there are upper class and Catholic. Parisians refer to them dismissively as “Versaillais”—conjuring an image of a khakis-and-Tod’s-clad churchgoer who reads the right-leaning Le Figaro more than Le Monde. In short, someone who is well off but provincial. While the conservative, pious nature of Versailles fit the Dupont de Ligonnès DNA, its social scene proved too suffocating for the flamboyant Bernard-Hubert. In the early 1970s, he abandoned the family and moved to Africa, leaving his son Xavier, around 10-years-old at the time, to be raised by his mother, Geneviève.

People who visit the property enter a chapel that displays the family coat of arms.

In some ways, Geneviève fit in among the Versaillais. She was very religious. But that’s where things start to fray. She was also the founder of a radical sect called “Philadelphia,” whose adherents believed the Pope was an impostor and that the entire Catholic Church had been diverted to the Talmud by Jews whose “Luciferian” plot would lead to a second French Revolution and, ultimately, the apocalypse. According to an article in France’s CNews, Geneviève went by the name “Violette” and claimed she received messages from “the beyond.” Philadelphians traveled to Brittany three times (once in the 60s and twice in the 90s) to await the future panic when the state would collapse and often stocked rations in their homes to last out the end times.

Within this odd mix—propriety in the parlor, doomsday lunacy in the cellar—Xavier seems to have developed two identities: the pious schoolboy who attended Mass each morning with his elder sisters before heading off to private school at the Collège Lycée Saint-Exupéry; and the aristocratic playboy who could be seen speeding around town in his Triumph Spitfire, seducing Versailles belles, women such as Agnès Hodanger, the daughter of a bourgeois Versailles family of lawyers and architects, who became his wife.

But like his father, Xavier never clicked with stuffy Versailles, and soon he and Agnès were on the move, living in various towns and cities such as Draguignan, Lorgues, Sainte-Maxime, Vaison-la-Romaine, and Pornic, growing their family along the way, before settling down in Nantes.

“Xavier didn’t want a normal life in Paris or in Versailles with a fixed job and an office,” a friend admitted in an article in Le Journal du Dimanche. “He wanted to move around like his father. He wanted to also make a fortune and dreamed of a big life, which in return would restore his family’s wealth.”

The four children of Xavier Dupont de Ligonnès, as seen in a family photo. From left: Thomas, Anne, and Benoît, with Arthur on their laps—aged 13 to 20.

More often, though, it was Agnès’s money that funded the family’s gypsy lifestyle, as Xavier’s various get-rich schemes failed to pan out—like his hotel guide for traveling businessmen, or a membership-card fidelity program linking clients to restaurants and hotels. Then there was an idea to help small businesses access different networks. The common traits among all of them? Few to no employees, meager profits, and the company headquarters often listed at Xavier’s home address.

“He wanted to also make a fortune and dreamed of a big life.”

It all made for a curious world. As a friend of Xavier’s 16-year-old daughter Anne confided to Le Monde, Anne didn’t really know what her father did for a living, only that he spent long periods of time in his basement home office working a lot.

According to the same article, Agnès complained in a 2002 e-mail to a childhood friend that Xavier had burned through her inheritance, and she didn’t know where to turn. Two years later, she posted on Doctissimo’s online forum: “Xavier is too judgmental, too quick to argue, too rigid, too military. There’s no more tenderness between us, no more attention, no softness, no sex. He hates when we tell him his behavior is unacceptable. He feels attacked and humiliated. And when I ask him if he’s happy, his response is the same. ‘Yes I am, but if we could all die tomorrow, that would be better.’”

In 2005, Agnès filed a police report claiming Xavier had hit her oldest son, Arthur, and in 2007 she sought help from a priest, citing religious differences between she and her husband, that the priest worried would lead to a “rupture.”

“There’s no more tenderness between us, no more attention, no softness, no sex.”

During this time, Xavier was reaching out to others, too. In e-mails to his sisters and to close friends, he described financial and psychological stress, insomnia, and panic attacks that lasted until noon. In 2010 he confided to a childhood friend with whom he had rekindled an affair that he’d hit “rock bottom” and that he needed a loan. In this eerily prescient e-mail, Dupont de Ligonnès admitted: “If this all goes bad, I’ll either blow myself up or set fire to the house.”

In the days and weeks leading up to the murders, Dupont de Ligonnès seemed to have followed up on that promise, charting a methodical path that would eventually establish him as the principal suspect. He inherited a gun and purchased a silencer. He practiced at a shooting range. He purchased heavy industrial shopping bags, cement, a spade and wheelbarrow. He also closed out one family bank account, canceled the lease on the house, and emptied much of its contents. Meanwhile, Agnès abruptly quit her job, claiming in a letter that she was moving to Australia, and on April 3 the family was last seen together dining out in a restaurant in Nantes, seemingly happy.

The Investigation

Since the bodies of the Dupont de Ligonnès family weren’t discovered until weeks after they were killed, the warrant for his arrest wasn’t issued until May 10. By then authorities were having to work backward in hopes of discovering his whereabouts. In early April, he was seen at the house in Nantes loading heavy bags into his car. On April 14, video surveillance showed he’d taken money from an A.T.M. in the South of France and later checked into a hotel. On the following day, he abandoned the car, which was discovered in the hotel parking lot on April 22. It’s there that the trail goes cold.

Pursuing leads, police searched a monastery; two people claimed they had seen Dupont de Ligonnès at Mass, mistaking a monk for Xavier. Another search led police to the Pic Martin mines near the town of Le Cannet-des-Maures in Var. Later, on CNews, Bruno de Stabenrath, a childhood friend of Xavier’s, claimed Xavier was living in Argentina, on account of his being fluent in Spanish.

Police interrogated monks at a monastery not far from the family château.

But the most outré theory may have come from Dupont de Ligonnès himself. On April 11, family members and friends received a curious letter, supposedly written by Xavier, in which he revealed that he’d been working with the D.E.A. “on an important international drug case” and that the family had to be immediately relocated to America under federal protection and that nobody could find them. “All I can tell you is that it’s warm and there’s good music.” The bizarre e-mail started with the line “Coucou tout le monde.” (Hey, what’s up, everyone!)

A police seal on the front door of Dupont de Ligonnès’s family home. A friend of Xavier’s 16-year-old daughter Anne confided to Le Monde that Anne didn’t really know what her father did for a living, only that he “spent long periods of time in his basement home office.”

Many who received his e-mail harbor hope that the other Dupont de Ligonnèses are still alive, that the bodies found in the garden aren’t theirs. Further proof for this theory may be in an envelope delivered to a journalist for the Agence France-Presse in 2015, which contained a photograph of Xavier with two of his sons and a handwritten message on the back saying, “I am still alive.” It’s not clear when the photo was taken or whether the letter was a forgery.

Even before the Glasgow snafu, the “XDDL” affair, as many call it, has proved to be the kind of conspiracy catnip French TV and the cinema adore. France 2’s show 13:15 recently speculated that John List, a man who killed his entire New Jersey family in 1971 before hitting the road and becoming a fugitive, may have influenced Dupont de Ligonnès simply because List’s arrest in 1989 and its subsequent broadcast occurred (get this) the same year Dupont de Ligonnès was living in the U.S.

A photograph of Xavier with his two sons and a handwritten message on the back saying, “I am still alive.”

Not long ago, the network TF1 aired the telefilm The Share of Suspicion, a movie inspired by the Dupont de Ligonnès murders and starring one of France’s biggest stars, Kad Merad. During the film’s promotion, Merad, who plays a man whose girlfriend comes to suspect he may have killed his entire family before meeting her, admitted in jest that he didn’t want the case to be solved before the premiere.

“I don’t know if we’ll ever learn what happened that horrible night, but I hope at least we don’t find out before the film comes out.”

Perhaps what’s most titillating about the Dupont de Ligonnès affair isn’t the grizzly nature of the murders themselves nor the off-type profile of the suspected killer, but the fact he disappeared into thin air, a quality in today’s age of surveillance and transparency and need for immediate closure that, to many, seems like spooky voodoo.

The one thing that is tragically certain is that Xavier Dupont de Ligonnès will have no descendants. And whether he’s dead or alive, or hiding somewhere in Scottsdale, Arizona, or on a yak near Chile, the centuries-old name that served his family as both blessing and curse will die with him, if it hasn’t already.

John von Sothen, an Editor at Large for AIR MAIL based in Paris, is the author of a memoir, Monsieur Mediocre