This movie was around 60′ / 70′ (I know, not exactly narrowing it out)
A detective is on the hunt of a serial killer around a big city, but is somewhat very elusive. The end of the movie lands him in the city sewer. With the volunteer help of a doctor, the detective reports to the outside crew his findings with a walkie-talkie, when they tell him they have a breakthrough on the description of the killer: “Suspect wears long brown trench coat, hat and glasses!”. That’s the exact description of the doctor right beside him in the sewer, staring at him with a big sadistic smile.
I had Walter Matthau as the detective but not so after a little search.