You drag a witness out who is willing to go on camera, in a magistrate's or other court, and swear then we'll listen.
Part of their reluctance may have to do with the fact that Zimmerman is on the loose and presumably still armed and dangerous. Which is a disturbing aspect ~
Go to the video I linked. The reporter says the police have a corroborating witness for Mr. Zimmerman on the vital point that Zimmerman broke off the confrontation and was heading back to his truck when Martin jumped him and started beating him. Not necessarily tetimony sworn in court, but pretty much the next best thing. Also, if the cops don’t identify that witness it will be hard for Zimmerman to hunt him or her down. Indeed, your characterization of Zimmerman as “armed and dangerous” appears highly prejudicial since Zimmerman hasn’t even been charged with a crime and, from the presently available information, it appears that, if there was any crime at all, it was Martin’s violent assault on Zimmerman.