Around 160 witnesses are expected to appear in the case The trial of four people behind an alleged art forgery scandal in Germany has got under way in Cologne. The group are accused of selling 44 fake paintings over the past decade, purportedly by 20th Century masters. Wolfgang Beltracchi, 60, is alleged to be the mastermind behind the scam which defrauded art collectors including the Hollywood actor Steve Martin out of millions of dollars. More than 160 witnesses are expected to appear over 40 days of the trial. Beltracchi\'s wife Helene, his sister-in-law Jeanette and her husband, Otto Schulte-Kellinghaus, were also arrested last year, accused of playing a part in the ploy. Paintings by the likes of Kees Van Dongen, Max Ernst, Max Pechstein and Heinrich Campendonk were faked, along with certificates of origin and labels. Scientific testing Many dealers, museums and art collectors were duped into believing the previously unknown masterpieces had been hidden for years by two secretive Cologne collectors and kept safe during the Nazi years. After art experts had been fooled into confirming their authenticity, it sparked a buying frenzy with galleries and auctions offering the works. But the forgeries came to light in 2008 after a buyer purchased what was thought to be a Campendonk through a Cologne auction house for 2.5 million euro (£2.2m) and had the work scientifically tested. The painting was shown to contain a colour that had not yet been invented at the time when it was painted. Hollywood actor Steve Martin found himself caught up in the scandal after he purchased another \"Campendonk\" in 2004 from a Parisian gallery. He later sold it on in 2006 before the forgeries were discovered. A judgement on the case is expected to be issued next March.