Police in Austria are urging fraud victims who may have bought forged Pablo Picasso paintings to come forward, Reuters has reported.

A press release published on the Austrian police website confirms that a gang of Serbian con-men were uncovered and arrested. The fraudsters allegedly offered fake Picasso paintings for around €300,000 and enticed potential victims by presenting forged certificates of authenticity. The transactions were conducted in a café in the Viennese district of Fünfhaus.

It is unclear how many people may have fallen for the counterfeits, and Viennese police has appealed to members of the public to come forward with information. According to the police statement, “The criminal police of Vienna assumes that other identical or similar transactions of forgeries have taken place and seeks potential victims to contact investigators.”

As Art News reported last year, Picasso’s works are often difficult to authenticate because the artist routinely neglected to sign his canvasses. To make matters worse, the committee set up by his five surviving children to authenticate works was disbanded in 1993 following a dispute over the authenticity of a set of drawings. The confusion culminated when two of his heirs began issuing certificates independent of one another. As a result of the murky certification process, Picasso’s artworks are often targeted by criminal gangs.