The Bird with the Crystal Plumage

In Italy, American writer Sam Dalmas witnesses a murder attempt on an art-gallery owner, Monica Ranieri, a few days before he's supposed to return home. Inspector Morosini, who is in charge of investigating the three previous murderers of the serial-killer, asks Dalmas for help 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 overhear a strange noise on the tape. Soon the serial killer stalks Julia and Damas. Who might be the killer?