I'm pretty sure that they have to preserve the evidence so the prospective grand jury members don't see it before a trial and have their mind made up before they see it in court. The ironic thing is that, in theory, this is done to protect the suspect so he has a better chance at a fair and balanced hearing. Yet the family members, conspiracy theorists, etc. are complaining about not seeing the evidence which could in turn hurt his chance at a fair and balanced hearing. Not too mention the fact that he hasn't even been charged yet. Does it really make sense to release evidence of a high-profile grand jury investigation to the public before the suspect is even charged?