Dirty Harry

A psychopathic killer, later referred to as "Scorpio", shoots a woman while she swims in a rooftop pool. He leaves behind a blackmail letter demanding he be paid $100,000 or he will kill more people. The note is found by SFPD Inspector Harry Callahan. The mayor teams up with the police to track down the killer, although to stall for time, he agrees to Scorpio's demand over Callahan's objections.