Are they waiting until everyone forgets about this? I for one am not forgetting.
They have the gun used to shoot the unarmed suspect to death, they know who the shooter is, they have the vehicle involved, the two witnesses in the back seat who watched their Dad get shot to death....Everyone and every thing is accounted for.
I would guess, they've had time to thoroughly investigate this incident, 20 times over by now.
Maybe they're waiting to see if an orbiting spy satellite happened to catch the event.