The Bird with the Crystal Plumage

In Italy, the American writer Sam Dalmas witnesses an attempt of murder of the owner of an art gallery, Monica Ranieri, a couple of days before returning home. Inspector Morosini, who is in charge of investigating the three previous murderers of the serial-killer, asks for help to Dalmas and takes his passport. Dalmas decides to stay with his girlfriend Julia and to help the police in the investigation. The killer threatens Dalmas and Julia by phone and the police overhears a strange noise in the tape. Soon the serial killer stalks Julia and Damas. Who might be the killer?