A Person of Interest may be someone who knows who did it but didn't do it themselves.
That person is an accessory after the fact. If they know who did it, but didn’t do it themselves. That, or they are a witness that has not yet come forward. Person of Interest is like person of color. Cops often speak of potential witnesses when they are trying to find aunt Bessie who was walking her poodle on the trail that day. But if they have the slightest scintilla of an idea that the homeless black guy running from the scene where the body was found might be involved, then he has a suspect. Not a person of interest.