Back in the days of the post 9/11 anthrax mailings, a new expression came into the parlance of law enforcement statements 'person of interest'. It was first used in reference to Dr. Steven Hatfill and other people under investigation for culpability in the anthrax mailings.
Later the term was used in describing John Muhammad during the investigation into the case of the DC area sniper. Again, it was a matter of investigating a person's possible responsibility for a crime.
A joke went that you could watch a car chase on TV, see the car finally stop, and the driver step out of the car. As soon as the police lay a hand on him, he suddenly becomes merely a "suspect". I think that is the type of situation that inflated the meaning of the word suspect. Now, people hear the word and it means "That's the bastard we're going to nail for the crime." Evidently, we needed a new word, or ungodly agglomeration of words, to describe a person that the police might want to nail for a crime.
Of course, don't you already think "The bastard whose going to get nailed for this crime" when you hear "person of interest"? I give it another six months before I have another post bitching about the new term to replace the loaded "person of interest".