George Zimmerman Update: Expert says screams on 911 call "were almost entirely those of Trayvon Martin"

(CBS/AP) After a pre-trial hearing that's spanned two days, a judge in the George Zimmerman case has yet to rule on whether the jury will be able to hear from voice recognition specialist, including an expert who claims to hear Trayvon Martin screaming and yelling "I'm begging you" in the background of a 911 call placed the night the Florida teen was killed.

PICTURES: George Zimmerman in court

READ: Trayvon Martin Shooting: A timeline of events

The screams captured on the 911 call is potentially crucial evidence since that could determine who the aggressor was in the confrontation. Martin's family contends it was the teen screaming, while Zimmerman's father has said it was his son.

Zimmerman argues he killed Martin during a confrontation last year in Sanford, Fla. in self-defense. His second-degree murder trial launches Monday with jury selection.

The testimony of the state's expert, Alan Reich, could prove key for the prosecution if the judge allows him to speak to the jury. Reich testified Friday afternoon via telephone.

Reich's analysis of the neighbor's 911 call picked up words that other experts never found, including the words, "This shall be" from Zimmerman and "I'm begging you" from Martin.

"The words at a screaming level were almost entirely those of Trayvon Martin," said Reich, who told the court he had spent "hundreds" of hours listening to the tape.

Reich said that his finding was "tentative." He said he couldn't find definitively whether the voice was Martin's because of the quality of the recording.

When asked by the defense if he wasn't able to reach a definitive conclusion, he said he held his opinion "firmly" that the screaming voice was Martin. However, he said "it can't be a hard and fast opinion because the circumstances are not ideal circumstances."

Tom Owen, another expert in voice identification, testified Friday that screams for help heard on the tape do not match Zimmerman's voice, though an FBI analyst who testified Thursday said the voices couldn't be conclusively identified.

Court is set to resume at 9:30 a.m. Saturday, when the defense is set to call three witnesses in the continuing hearing.

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