The police chief said today that he had turned the investigation over to another LE agency. Might be the FBI, who have officially opened a civil rights investigation into the matter. Anyway, that may explain the delay - that or toxicology results, which take weeks.
Well, I can assure you, regardless of which agency is conducting the investigation, they already know what happened at the scene, based on substantial physical scene evidence alone. Plus the officer involved statement etc. etc..
This should not be complex with this much physical evidence...It's a shooting in the middle of the street, with spent casing, gun shot residue, blood, a body, wounds, the gun, the shooter and witnesses.
In addition, due the fact they have substantial investigative resources available due to the expanding scope of this event, this investigation should be near completion.