Whodunit where a murder hidden among a serial killer's victims is uncovered, the...

I'm trying to recall a crime movie or TV show made (and probably set) in the 20th century, where there's a serial killer with several victims. Our perceptive protagonist figures out that one of the crimes is subtly different. A previously unsuspicious character did it, trying to foist it on the first killer.

Unfortunately, I don't remember any specific details. Or more precisely, I'm sure I remember some of them, but they're blurred together with my memories of other mysteries & thrillers. Could be anything from film noir to a cop show.

It had more plot than one could reasonably fit into an hour, so probably a movie, but possibly a series with continuity. And I think it was a whodunit, with internally consistent clues.

I saw it in the 1990s, on video, not in a theater.

Maybe Red Riding: In the Year of Our Lord 1980, (2009).

With a serial killer claiming victim 13 and rumors of corruption in their force, the West Yorkshire cops are told to cooperate with a team from outside - Peter Hunter and two hand-picked associates. Hunter gets little help but plunges ahead, discovering that one of the 13 victims may have a different killer. This part of the investigation leads to late-night calls, another murder, and bureaucratic moves to push Hunter aside: he may be getting close, not to the serial killer but to bad apples in the force.