I am curious as to what this term means. Obviously, I know that they are a suspect in a crime, but why is it that the police will not or cannot prosecute them? Especially if they are the only "person of interest" in a given case?
I can understand that sometimes the evidence is a little flimsy, or that correct procedures have not been followed (I have watched a movie or two).
But for a "Person of Interest" to wander around, or even be in gaol (jail to my American friends). It makes no sense. If anything it seems to mock Justice.