According to the article, this scheme was probably just a fabrication by a journalist who wanted a juicy story:
"Here at last was a criminal brain worthy of the Mona Lisa. The only problem is that Decker almost certainly invented him. There is no external evidence for Decker’s story, nor even for the Marques’ existence. A century later, none of the six supposed copies has surfaced. Most likely, Vincenzo Peruggia stole the Mona Lisa single-handed, largely because she was small."
(Also, this is somewhat off-topic, but the typography and layout on ft.com are really superb. This is one of the few websites that doesn't make me immediately turn to readability.)
"Here at last was a criminal brain worthy of the Mona Lisa. The only problem is that Decker almost certainly invented him. There is no external evidence for Decker’s story, nor even for the Marques’ existence. A century later, none of the six supposed copies has surfaced. Most likely, Vincenzo Peruggia stole the Mona Lisa single-handed, largely because she was small."
(Also, this is somewhat off-topic, but the typography and layout on ft.com are really superb. This is one of the few websites that doesn't make me immediately turn to readability.)