London—When the British Museum’s antiquities expert first saw the Assyrian artifacts in 2005, he could hardly believe that such a rare and important discovery had landed on his desk. The owner was a middle-aged man with a shy manner and a credible story of inheriting the works from his great-grandfather.

"Initially, the work looked genuine," says John Curtis, who heads the museum’s Middle East Department. The three reliefs were ancient battle scenes intricately carved into cracked stone, and closely matched a drawing in the museum’s collection. But before handing over the asking price of $1 million, Curtis asked two specialists from Bonhams auction house for their opinion. The experts soon agreed that the artifacts were fakes and called in Scotland Yard. "The provenance was convincing, but the surface, the stone color and the patination were wrong," says Chantelle Roundtree, head of antiquities at Bonhams, referring to the largest relief. "It just didn’t fit the story that the piece had been hidden in their garage for decades."

The museum’s tipoff sparked an investigation that broke up one of the most prolific forgery gangs in history, the Greenhalgh family: 47-year-old Shaun and his octogenarian parents, Olive and George. All three have since admitted to defrauding art institutions between 1989 and 2006, and Shaun Greenhalgh was sentenced to four years and eight months in jail for the production and distribution of forged art. His mother received a 12-month suspended sentence, and at press time his father was scheduled to be sentenced on January 28.

In March 2006, when the police finally raided the Greenhalgh home, a three-story red-brick building in a housing project in Bolton just outside Manchester, they found half-finished sculptures, blocks of stone, piles of art books, silver for melting and a bust of Thomas Jefferson in the loft. The backyard shed and the galley kitchen served as Shaun’s studio, where for 17 years he created fakes that fooled museums, galleries and auction houses in Britain and the U.S. His father was the front man, concocting elaborate and meticulous provenance details.

The Greenhalghs are known to have passed off at least 120 fakes with a potential face value of $20 million, and made around $1.6 million from their activities. However, the police say, they lived frugally. The Greenhalghs never even owned a computer and did most of their provenance research at the local library.

"Shaun Greenhalgh didn’t do it for the money," said Detective Sergeant Vernon Rapley, who heads Scotland Yard’s art and antiques team. "It really was an unbridled hatred of the London art market. It was his ambition to show how corrupt it is and how easily deceived art buyers can be." Whether the forger’s primary aim was to embarrass the art market remains unclear, and he isn’t talking. But it is clear that he has an extraordinary gift for reproducing other artists’ work. Using old photographs and sketches, Greenhalgh copied the works of L.S. Lowry, Barbara Hepworth, Horatio Greenough, Paul Gauguin, Man Ray and Constantin Brancusi. His forte was the work of the 19th-century American landscape painter Thomas Moran, who spent several years in Bolton.

He was able to work in almost any genre, from painting to carving, and to copy almost any period from antiquity to the present. Before presenting the pseudo-Assyrian relief to the British Museum, the Greenhalghs sold a purported Egyptian marble torso known as the Amarna Princess, which they claimed was 3,300 years old, to the Bolton Museum for around $850,000. A sculpture attributed to Gauguin called "The Faun" was bought by the Art Institute of Chicago for $350,000 at Sotheby’s New York in 1997. It was also a Greenhalgh forgery, and the museum is now discussing compensation from the auction house. A number of Greenhalgh’s Moran watercolors that also came under the hammer at Sotheby’s have yet to be recovered.

How did the Greenhalgh family get away with their scheme for so long and dupe so many experts? Their secret was a rare combination of masterful artistry and convincing stories. "Shaun Greenhalgh was able to recreate virtually anything, with incredible accuracy," says Detective Constable Ian Lawson of Scotland Yard. "The father was a master of creating provenance, which was built up over a long time by careful research and writing the experts. The parents were also very good at selling the artwork. Who’s going to believe that two old dears are master criminals?"