professor Law enforcement isn't really designed as crime prevention
In these situations that’s point (visibility). Law enforcement is visible and is intended to dissuade criminal behavior. We don’t know the details yet.
But my guess is law enforcement was there rather quickly, hence the quick apprehension. And I’ll bet the shooters had some sort of gun that could fire rapidly.
Also reports are this was some sort of argument that turned to gunfire. If that’s the case they sucked at aiming. They missed shooting each other, but hit two dozen innocent people.
Again, all speculation. But it’ll be interesting to hear what actually took place here.