The lead investigator and prosecutors reaction is typical. My wife worked for the prosecutors office here in Cbus and she's pretty clear that when something like this happens, everyone is out to find someone to blame. They most certainly were not in any way going to try and clear Zimmerman.
So their not believing him and having an itch to move forward is typical and very much a reality. If they didn't because there's lack of evidence or in this case what they claim is evidence to the contrary, it must have been solid.