George Zimmerman Not Screaming for Help, Two Experts Testify Today

June 7, 2013— -- Two forensic experts testified today that they believe the voice of someone screaming for help moments before Florida teenager Trayvon Martin was shot dead belong to Martin and not to George Zimmerman, the man accused of murder in Martin's death.

But the legal arguments over the tapes will continue into the weekend as prosecutors and defense lawyers wrangle over the testimony of experts who disagree over who is screaming and whether the technology exists to identify the voice.

Judge Debra Nelson is being asked to rule on whether the tapes can be admitted as evidence in the murder trial that is scheduled to begin Monday with jury selection.

The screams were heard in the background of 911 tapes as people called in during the lethal confrontation between Zimmerman and Martin.

Lawyers for Zimmerman claim the voice belongs to their client while prosecutors insist it is Martin's last words before he was fatally shot on Feb. 26, 2012. Determining who was that voice could make or break Zimmerman's assertion that he shot Martin in self defense.

During the pretrial hearing forensic experts disagreed over who is doing the screaming on the tape and an FBI expert testified earlier this week that it is impossible to determine which man was yelling for help.

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The state called two witnesses today who said it was Martin pleading.

Alan Reich, who said he spent hundreds of hours listening to the audio, said he could hear two voices. He said Martin could be heard yelling "Im begging you" and "stop." He says Zimmerman could also be heard, but he was not pleading for help.

Reich said he used voice samples from both Zimmerman and Martin to help him with his findings.

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Earlier, Tom Owen, also called by the state, testified he also did not believe the screams were from Zimmerman.

"It is much easier to eliminate someone than identify someone," said Owen, an audio forensic expert with over 50 years of experience. "The screams don't match at all with Zimmerman's samples."

ABC News has exclusively obtained a sample of Martin's voice and sent the short sample, gathered as evidence from Martin's cell phone, to a forensic analyst. Kent Gibson of Forensic Audio tells ABC News that a comparison of Martin's voice, Zimmerman's voice and the screams on the 911 tape, indicate the voice is more likely to be Zimmerman than Martin by a significant margin.

However, he adds, so much of the howling and pleading overheard on that 911 tape is muffled or obscured, that only two seconds of the tape are useable. Therefore, he says, that there can be no definitive identification of "the screamer."

The FBI's leading forensic audio expert said much the same in his testimony Thursday. Dr. Horotaka Nakasone called it "disturbing" that someone would be able to make a positive voice identification based on the screams.

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Nakasone testified that dissecting the 40-second 911 sample and figuring out who exactly screamed couldn't be done because only three seconds were unobscured.

"[The sample] has to be at least 16 seconds long," said Nakasone. "American English has 44 different sounds. To cover all those it takes 20 to 30 seconds."