Without Apparent Motive (1971)

Very good adaptation of Ed McBain's 87th precinct crime novel about mad sniper who is gunning down people on the french riviera. Detective Carella (good performance by Jean-Louis Trintignant) is questioning friends of gunned people and slowly finds clues, while at the same time encountering pretty chicks, and in the end it turns out all those people had a sex party long ago where they molested some girl who is now mental case, and her father takes justice in his hands by killing them off one by one.