The Bird with the Crystal Plumage (1970) on Netflix

Sam, an American writer in Rome, witnesses a murder attempt on the wife of the owner of an art gallery by a sinister man in a raincoat and black leather gloves - but Sam is powerless to do anything as he gets trapped between a double set of glass doors in going to her aid. The woman survives, and the police say that she is the first surviving victim of a notorious serial killer. But when they fail to make any progress with the case, Sam decides to investigate on his own, turning up several clues that point in the direction of just one possible suspect - assuming that he really knows who he's looking for...