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George Zimmerman trial: Voice expert testifies during second week of testimony

State witness Dr. Hirotaka Nakasone, a senior voice recognition scientist with the FBI, testifies in the George Zimmerman trial in Seminole circuit court, July 1, 2013 in Sanford, Florida. Photo by Joe Burbank-Pool/Getty Images

(CBS/AP) It's not possible to scientifically identify who is screaming in the background of a neighbor's 911 call placed to dispatchers the night of the fatal confrontation between George Zimmerman and Trayvon Martin, an FBI audio expert testified at Zimmerman's murder trial Monday. However, a person who is familiar with Zimmerman's voice or Martin's voice might have a better chance of identifying it, according to the expert.

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Circuit Judge Debra Nelson took into account the pre-trial testimony of FBI expert Hirotaka Nakasone and two other defense experts when she ruled that a state witness who said Trayvon Martin was screaming in the background of the call could not testify at trial. The ruling didn't bar the jury from hearing the 911 call, which has already been played in the courtroom.

The 911 call was the subject of much pre-trial wrangling and is crucial evidence in the case, because it could provide clues to who was the aggressor in the confrontation.

Even though he was a pre-trial witness for the defense, prosecutors called FBI expert Hirotaka Nakasone, possibly to set up later testimony from either Martin's mother or father that they believe it was their son yelling for help. Zimmerman's father has also said he believes it was his son.

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Nakasone took the stand Monday morning as the second week of Zimmerman's second-degree murder trial launched in Seminole County, Fla.. Zimmerman, a former neighborhood watch volunteer, is charged with fatally shooting Florida teen Trayvon Martin during a confrontation in a Sanford, Fla. gated community.

Repeating some of his pre-trial testimony in front of the jury, Nakasone said that the length of the screaming voice that wasn't "stepped on" by other sounds - a little less than three seconds -- was too short to reliably use for identification purposes. That the screams were uttered in an "extreme emotional state" also made them difficult to compare to normal speech, Nakasone said.

It was also impossible to determine the age of who was screaming, Nakasone said.

Nakasone said that the sample was "not fit for the purpose of voice comparison." However, he said the "best approach would be familiar voice recognition by an individual who has heard him ...speaking uttering in a variety of conditions including screaming, yelling under similar setups."

Complete coverage of the George Zimmerman Trial-Trayvon Martin case on Crimesider

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