Who Killed the Prosecutor and Why?

Original title: Terza ipotesi su un caso di perfetta strategia criminale

Carlo, a young photographer, is making love with his girlfriend at the beach when he witnesses a murder being feebly staged to look like a car accident. Carlo takes incriminating shots of the murderers (the victim turns out to be the local public prosecutor) and attempts to sell them to various interested parties, including a gangster and a newspaper owner. One of the murderers' accomplices, a lowlife criminal, is gunned down by a black-gloved assassin, while another is stabbed to death...