The trial of four people behind an alleged art forgery scandal in Germany began in Cologne on Thursday.

German authorities claim that the group is responsible for a multi-million-dollar art scam with victims that include actor Steve Martin.

Art experts, Christie's auction house, reputable Parisian galleries, and collectors were duped by the fake paintings with fabricated provenances slowly seeped into the market by the art forgery ring, possibly since the 1990s.

Martin had purchased a bright landscape by German Expressionist painter Heinrich Campendonk from Parisian gallery Cazeau-Béraudière, for €700,000 ($850,000) in 2004. He sold it in 2006 at Christie's for a loss and before the forgery ring was revealed.

Now Martin's previously-owned painting has been deemed a fake, one of 44 forgeries of early 20th century modernist painters such as Kees Van Dongen, Max Ernst, Max Pechstein and Heinrich Campendonk identified so far.

Certificates of origin and exhibition labels were also faked. An unknown collector who hid the paintings during World War II was also made up.

Wolfgang Beltracchi, 60, is alleged to be the mastermind behind the scam. His wife Helene, his sister-in-law Jeanette and her husband, Otto Schulte-Kellinghaus, were also arrested last year.

The forgeries were exposed in 2008 after a buyer purchased what was deemed a Campendonk through Cologne auction house Lempertz for 2.5 million euros.

The buyer had the work scientifically tested which showed the painting contained a pigment that had not yet been invented at the time when the artist would have painted it.

Since then 14 other works traced to the accused forgers have been tested and found to be fake.

More than 160 witnesses are expected in the trial.

A judgement in the case should be made by next March.