You will not be able to find a DOJ, or court established definition of Person of Interest. It’s nothing more than a politically correct term for suspect.
Statistically, most persons of interests are likely suspects, that is true. However, as the law uses the term, it just means somebody who may have information leading to the arrest of the suspects, even if that information ends up implicating the person of interest themselves.
But it is not a legal term, that is also true.
“Person of interest” is a term used by law enforcement in the United States, Canada, and other countries when identifying someone possibly involved in a criminal investigation who has not been arrested or formally accused of a crime.[1] It has no legal meaning, but refers to someone in whom the police are “interested”, either because the person is cooperating with the investigation, may have information that would assist the investigation, or possesses certain characteristics that merit further attention.[2]
https://en.wikipedia.org/wiki/Person_of_interest