They cuffed Zimmerman, took him into custody and took him downtown for questioning, entering his weapon into evidence.
They immediately turned the scene over to major crimes to investigate.
They canvassed the neighborhood, taking statements from all witnesses. None of them contradicted Zimmerman’s account.
I’m not a LEO. What steps were missed?
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Well, it would seem that testing Zimmerman for drugs/alcohol and testing his clothes for blood splatter and testing Martin's clothes for GSR were not done.