Man on the Roof (1976)

The ancestor of everything else here. Bo Widerberg's Martin Beck film opens with a police inspector murdered in his hospital bed, and the investigation turns up complaints of misconduct against the dead man, so the real question becomes who in Stockholm had reason to hate a policeman. Carl-Gustaf Lindstedt plays Beck as a tired, decent functionary rather than a hero, and Widerberg shoots the city in a flat, almost documentary way that makes the legwork feel real. The social anger that runs through the later TV shows is already fully formed here. Start with this one to see where the genre's habit of investigating the institution, not just the killer, comes from.