Let's examine the means, motivation, and opportunity of the shooting to narrow it down who coulda dunnit:
Means: Just some generic pistol that anyone who isn't a felon could legally buy (and felons could illegally buy), subsonic rounds that (almost?) everyone could buy, and a silencer that could be legally purchased or illegally manufactured.
Motivation: Literally anyone who has ever been insured by United Health.
Opportunity: Anyone who was in NYC on that day.