Verona, January 21 - Verona prosecutors are investigating Pietro Maso, a 44-year-old who served 22 years for killing his parents in 1991 with the complicity of three friends, for alleged extortion against his sisters, ANSA sources said Thursday. Il Corriere del Veneto reported that the probe was opened after the sisters, Nadia and Laura, filed a complaint against Maso, who was released in 2013, for allegedly demanding money from them. The double murderer was in the headlines earlier this week for saying that Pope Francis had replied to a letter from him asking for forgiveness. Maso was just 19 when he donned a Carnival mask and ambushed his parents Antonio Maso, 52, and Rosa Tessari, 48, at the entrance to their home and battered them to death in order to come into his inheritance. He was aided by his friends Giorgio Carbognin and Paolo Cavazza, both 19 at the time, and 17-year-old Damiano B., all of them also donning Carnival masks. The four killers then left their victims in a pool of blood and went dancing at a nightclub.