Well let me rephrase that. I'm assuming that the police investigated the shooting and that they recorded where the shell casings were. But that information hasn't been released.
And there may be a perfectly good reason. Especially if Wilson is charged with anything. That info should be part of the (decision on whether to charge him ?..... nah) trial.
HOWEVER the media and certain minority support groups (CBC,NAACP,BLACK BLOC,FBI,DOJ, and the President) are using the 'absence' of evidence to twist the view of the public.