George Zimmerman Trial: Trayvon Martin's Father Testifies

"Nightline's" Dan Abrams offers his take on the Florida prosecution's murder case.
3:00 | 07/09/13

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Transcript for George Zimmerman Trial: Trayvon Martin's Father Testifies
Today in Florida George -- attorneys presented their first full day of witnesses in his murder trial. Throughout the trial simmons' friends and family hadn't called to the stand not just to say that they knew him -- liked him. But that they recognized his voice on critical pieces of evidence those disputed 911 calls it a case filled with he said she said -- Another twist. The first testimony from trade on Martin's father. Comes during the defense case in the trial of George -- -- -- George Zimmerman it's one of the most crucial questions in the George Zimmermann case. During certain -- who was heard yelling for help on the various 911 calls placed that night. But he didn't really critical. Look I'm able I'm not sure there's usually standing upright. Yeah. Yeah. Prosecutors argued the voice was that of seventeen year old -- -- -- shot once in the chest and killed after a confrontation with Zimmerman last February. Now all eyes and years are on those 911 -- which could help show who was the aggressor who was on top. Who was beating who before the fatal shot was fired and was this self defense says Zimmerman claims. -- -- you today witness after witness friends of George Zimmerman were called by his defense team to testify. They are confident it was him Zimmerman heard screaming on those tapes. Do you know whose voice that is in the background -- -- and here I hear him -- spoon. I don't storage resources -- Herman's voice there is absolutely no doubt in my mind. -- -- person. Who wish to god it did not have a better feel and understand. This comes after the mothers of both Morton and Zimmerman took the stand late last week. Both calmly and confidently. And definitively identified the voices that of her son on the 911 tapes. Didn't recognize that to be. Turn on Benjamin Martin when I'm Jason Douglas to -- displays. But what about trade -- Martin's father Tracey Martin. He -- testifies part of the prosecution's case -- BC news and today the reason for that may have become clear. According to two police officers including the lead detective. Two days after the incident Tracey Martin heard the 911 tapes for the first time and said he did not believe the voice on the tapes was -- -- I act requires that that was effects -- don't know -- horses. He looked away and under his breath -- terrorists that -- Traci -- and doesn't remember it that way. In a bold and some might -- risky move the defense then called trade bonds' father understand -- -- congress is whether cancer cancer. To viewers through IQ -- something that affect weather -- adversary. No wasn't so voice. He now says after hearing the tape twenty or more times he's convinced he was trade -- cries for help. On those tapes. I was -- so. My son's Blair's preference known most listened to his life being taken. I was coming to grips tree while I was -- -- -- The defense wants the jury just a focus on the 911 call and the screens being hurt because of those -- are George Zimmerman then. The jury may find that he was acting in self defense. The government -- wants this jury to focus on who started this confrontation who was the initial aggressor. In a blow for the prosecution today judge Debra Nelson ruled this government's lawyers can introduce evidence -- on Morton had marijuana in his system when he was killed. They say it supports -- call to police sources. Follow him where he reported that person spotted you know looking like he was on drugs. This guy looks like he does no good -- is under I was at home. Medical examiner shipping ballot initially reported the amount to load to affect -- but later changed his assessment. Today the defense also called mixed martial arts trainer Adam -- to the stand. He briefly worked with Zimmerman but said his client couldn't throw -- each theater progress report -- -- could -- somebody else. It's absolutely not he still learning how to punch. He didn't know how to really effectively punch. The defense trying to bolster its case. -- Zimmerman would never been able to have beaten pummel Martan.

This transcript has been automatically generated and may not be 100% accurate.

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