Lino Ventura, M. Serrault and Romy Schneider. What else can you say? And Guy Marchand (from the seedy detective series "Nestor Burma"). The script flows like a clockwork orange, of course aided by Serrault, who could make rocks weep, laugh and commit suicide. Lino Ventura is believable, almost likable, as a tough cop who "doesn't really care who gets nailed" but can't forget the young victims, and won't be distracted from a suspect even if he's got style and wit.
Why does the ending disappoint? Probably a tad melodramatic and manipulative, it doesn't harm an already superb film. It only makes it far from perfect. I agree with IMDb's reviewer "Taylor, from Ottawa" that "night scenes in a steady rain" by Bruno Nuytten create the perfect oppressive if somewhat awkward climate, necessary for this theatrical plot to unfold. Enjoy, if possible on a damp night :)!
PS: The remake "Under suspicion"... Hackman is fine, but can't reach Michel's subtlety and yet how disagreeable he can be; he seems born for this "gifted neurotics" roles. Monica Bellucci is nice, but cannot carry the nostalgia Schneider exudes. W. Chisholm's review on Amazon is right she shows Visconti's training. Adaptations should only be made when improving on the original, don't you think :)?