phil. reporter: a strategic move by prosecutors, or jenna, rather, by prosecutors as they called one of the defense s audio expert witnesses first to then set up the still to come very emotional testimony anticipated by trayvon martin s mother. inside the courtroom right now sanford police detective doris singleton. she was involved in one . very first initial interviews with george zimmerman the night of the shooting. again today testimony addressing that infamous 911 call with somebody screaming in the background. stkpwhr-f 911 do you need police, fire or medical. maybe both i m not sure. there is just someone screaming outside. reporter: prosecutors called fbi audio expert who concludes only three seconds of that call is fit for speech analysis and that scientifically the screamer cannot be identified. prosecutors got him to say that