High above the Encino Reservoir, streets curl like ribbons atop the Santa Monica Mountains. Evergreens shield the well-tended lawns from prying eyes. On a sleepy afternoon in February 2006, a white pickup rolls to a stop near the peach-colored house where a prosperous electronics importer lives with his wife and their son. The truck is an ordinary 1992 Dodge. There's nothing unusual about the driver who exits the vehicle and moves toward the family's home. He is clean-cut and darkly handsome, with intense eyes and the physique of a middleweight boxer. He wears running shoes, board shorts, and a sun visor. He could be a contractor, a gardener, or a houseguest. He could pass for a man in his twenties or thirties. He could be just about anyone, and that is precisely the impression he wants to leave.

Other cat burglars may operate at this elevation, but none can approach this man's level of expertise. One month earlier, the "Hillside Burglars" began a three-year run that will expand to include a reported 150 jobs around Bel Air and Beverly Hills, and the teenage "Bling Ring" will gain notoriety three years later for stealing some $3 million in property from celebrities like Paris Hilton and Lindsay Lohan. Those thieves are rank amateurs compared to this guy. In 16 months, he has broken into more than a thousand homes up and down the San Fernando Valley. According to the police, his haul is worth anywhere from $16 million to $40 million. And yet because he has cultivated so many aliases, law-enforcement officials have been hard-pressed to learn his real name—Ignacio Peña Del Río—much less comprehend his unlikely background.

Del Río can easily scale a second-story balcony. For greater heights, he uses a homemade grappling hook. He is a virtuoso lock picker and an expert at defeating alarm systems. He knows how to cut phone lines and confuse motion detectors. In the bed of his Dodge pickup are some of his tools: drills, bolt cutters, angle grinders, a sledgehammer, a blowtorch, a glass cutter, a railroad pick, a handheld cable puller that can move four tons, a hydraulic jack that can lift six. For a job that requires surveillance, he brings a DVR with miniature cameras and a wireless monitor. He rips safes from walls; he steals artwork, heirlooms, gold coins, silver ingots, wedding rings, diamond-encrusted chokers, a Cartier Panther brooch, even a bronze medal from the 1984 Summer Olympics—whatever he can grab in the few minutes he's inside a house. He hoards his loot in a storage locker off Ventura Boulevard, existing at once as a hermit and a high roller: He lives in a ratty UPS-style box truck that he owns and showers at a 24-hour health club yet dresses in Calvin Klein and Burberry. Del Río doesn't need to play this game. The great ones never do. Ordinary thieves steal for money. The best do it for sublime reasons: to exercise a forbidden expertise, to undermine society, often just to feel alive. Some, like Del Río, adopt a Robin Hood philosophy (although they rarely get around to helping the poor). We deplore their crimes even as we romanticize their genius. See Cary Grant in To Catch a Thief. As Sherlock Holmes once said of Charles Peace, the legendary cat burglar of Victorian England, in the short story "The Adventure of the Illustrious Client": "A complex mind. All great criminals have that." Indeed, the first reaction from investigators who capture elite burglars is generally not reproach but grudging admiration. The cat accomplishes what the rest of us can only imagine. He is an artist working in adrenaline.

Like other top-drawer burglars, Del Río shuns drugs and alcohol and keeps himself in impeccable shape, exercising for hours every day. He fits the profile: a gifted obsessive who displays a lust, even a primal need, for action. As he approaches the peach-colored home in Encino, he knows that Detective Bill Longacre of the LAPD is hunting him and closing fast. This does not deter him. As if this were just another Friday, the city's most accomplished thief slinks through a bedroom window and into crime lore.

HE WAS BORN INTO AN AFFLUENT, SUPPORTIVE FAMILY IN ASTURIAS, SPAIN, IN 1974. One of his sisters is a lawyer. Another is an architect, as are his brother and his father. As a teenager, Del Río attended the Colegio de la Inmaculada, a prestigious 120-year-old Jesuit boarding school in Gijón that counts the billionaire banker Emilio Botín among its alumni. In 1992, the young Spaniard left home for Michigan to participate in a high-school exchange program, and there he began his rap sheet: He was busted in Ypsilanti for shoplifting spark plugs. A year later he was arrested in Gijón for a burglary. He avoided jail by paying a fine and, in 1998, returned to the United States. Using credits from a community college in Madrid, he enrolled in the University of San Diego. He loaded up on classes, graduated a year later with a business degree, and headed for L.A. to pursue his dream of becoming a professional fighter.

Del Río had studied kickboxing since childhood. He could unleash a foot at head level as easily as a fist. For a time, he bounced around the local fight scene, working, he says, as a sparring partner for WBF lightweight champ Juan Lazcano at Freddie Roach's Wild Card Boxing Club. He practiced Brazilian jujitsu and trained for his pro-boxing debut, a fight he won by majority decision at the Hollywood Park Casino. But he soon committed his considerable talents to another pursuit. As Del Río sculpted his body, he began counterfeiting his life.