I could be wrong, but this sounds like a Mob/ Contract Hit. I wonder if the police dug really deep into his activity background, especially the past. Also, affiliations within his circle. If an individual is gunned down in public, around multiple witnesses, but no one claims to have seen anything, what does that tell law enforcement? DANGER! Therefore, nobody is going to talk if they know what’s good for them. Which also could mean they can not only describe everything, but tell you who the shooter was. Situations like this, no one will put their family in that kind of danger.
Believe it or not, the ONLY chance law enforcement has of solving some of these cases is to offer a large reward in these hard times for people, and promise them anonymously upon conviction.