OK, so, as I try to work it out in my head, it seems that we could potentially come up with an answer here ourselves.
A POI is someone that LE believes "knows something," I think.
A suspect is someone they believe committed the crime.
So when an article says:
"The persons of interest, Bailey said, know what happened to Drexel.
"They're suspected of being present with Brittanee, knowing her whereabouts or possible whereabouts," Bailey said."
I take that to mean that LE knows they were with Brittanee on the night of her disappearance, but they can't prove they committed a crime. In knowing they were with her, it seems to me that they might either have camera footage (unreleased) or eyewitnesses.
Does this information follow so far?