From The Guardian:
"..."The letter was accompanied by piece of the painting, a tiny piece of the canvas, which was intended to make clear to me that they really had the Caravaggio in their possession," Benedetto told his interviewer. "I went straight to the superintendent and informed him of what was happening. I left him the letter and the piece of canvas."
"The mafia was doing with the painting what they normally do with kidnapping victims", says D'Anolfi, who, at 45, is now an acclaimed director and will be screening the full interview next month in Palermo. "They had sent a piece of the painting just like they normally send a finger or an ear of a kidnapping victim."