Jon's already given you some very good detailed feedback, and I'm a novice writer so I wouldn't want to steer you wrong on any technicalities, so I'll provide some general comments as a reader:
I read this 'cold' without paying any attention to the genre or to the other comments. The first part was interesting, and I got a good impression of the busy street before Christmas. I was confused as to why the cop was so concerned about the boy being on his own, because where I live, in that decade even little kids were allowed to roam around all day without any supervision. Maybe you could hint at the cop's gut feeling that something is wrong a bit earlier in the story, or maybe the boy is very young or looks particularly vulnerable.
There are occasional phrases that seem a bit out of place, though I lack the experience to tell you why, and this might just be the way my brain works! examples are 'milling throng' and 'inordinate amount of time'.
I started paying more attention when the FBI was introduced - and I realised that this is my kind of story :). I'm interested to know what happens next.
Also, hello from a fellow Engineer (and I am not far behind you on age). I'm in aerospace - been an odd few months, given how the pandemic has affected the fleets.