Believe it or not, when a grand jury spends nearly a full month interviewing over 60 witnesses and all the physical evidence the can find and release a detailed 4,800 page document containing all the testimonies and evidence, they generally don't reverse their decision due to a few politically motivated tweets.
That said, this was only one of several investigations. The FBI is independently investigation this as a civil rights case, and the local prosecutor has nothing to do with that one.
http://www.youtube.com/watch?v=uj0mtxXEGE8
(Language warning)