Sea of Love (1989) on Netflix

In Manhattan, Detective Frank Keller is an efficient and veteran cop with twenty years on the force, who is lonely and misses his ex-wife Denise. She left him to live with his colleague from the same precinct Gruber. When a killer kills his victim with a shot on the neck and leaves a single playing "Sea of Love", Frank is in charge of the investigation. Detective Sherman, from a district outside Manhattan, joins Frank when a second victim is killed in the same MO. They find that both victims have written poetry in the lonely hearts column of the single magazine "New York Weekly". When there is a third case, Frank convinces his chief to write for the magazine and work undercover investigating the women that reply the advertisement. Frank falls in love for the prime suspect, the manager of a shoe store Helen Cruger, jeopardizing his investigation.