Well that was...interesting.
My take is that they believe it is someone she knows and they do not believe the public is in danger bc of that. But if they say "no danger to the public," that becomes very obvious and they want to play close to the vest.
Also the wording of "Finding out what is normal for Mollie, what isn't, and who was in/near Brooklyn on the 18th" make me think this is someone she knows maybe who doesn't live in Brooklyn necessarily or was supposed to not be there on the 18th (I am NOT pointing fingers at bf, housemates, etc. She probably knows a LOT of people who had no reason to normally be in Brooklyn on the 18th, even college buddies). And maybe they have reason to believe she was with someone or doing something that night that was not the norm for her--random meet-up with a college friend or co-worker, etc. kind of thing. I don't know, JMO from hearing all this.