Wow that is a coincidence, I saw that was up streaming on amazon prime and just watched that on impulse a few days ago. I hadn't seen it for many years. Terrific movie. Really reminiscent of Fritz Lang's M, so much so that I went ahead and bought the M blu ray a day later, on Saturday, because Citizen X put me in the mood. Both terrific movies, and easily a couple of the best films made about serial killers.
I will add that one of the best first sentences of a movie review was
a review for Citizen X: