SANFORD, Fla. (AP) - An FBI audio expert whose pretrial testimony helped keep prosecution witnesses from testifying at George Zimmerman's murder trial took the stand Monday and said a person who is familiar with a voice on a recording has a better chance of identifying it.
The issue is who was screaming for help on 911 calls during the confrontation that ended with the neighborhood watch volunteer fatally shooting 17-year-old Trayvon Martin. Jurors were played the 911 calls several times last week.
The recordings are crucial pieces of evidence because they 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.
Even though he was a pre-trial witness for the defense, prosecutors called FBI expert Hirotaka Nakasone to set up later testimony from either the teen's mother or father that they believe it was their son yelling for help.
During his pre-trial testimony, Nakasone testified that there wasn't enough clear sound to determine whether Zimmerman or Martin was screaming on the best 911 sample, an assertion he repeated Monday.
"It's not fit for the purpose of voice comparison," Nakasone said.
Nakasone also said guessing a person's age by voice is "complicated" in general, and it was impossible to determine with the 911 sample he heard.
The FBI expert said that it's easier for a person with a familiarity of a voice to identify it than someone who has never heard it previously. "It's better," he said.
But under cross-examination by defense attorney Don West, Nakasone said there was a risk of increased listener bias if people trying to identify a voice are listening to a sample in a group, as Martin's parents did, rather than individually.
"There might be a risk of bias included in the end results," Nakasone said.
Nakasone's pretrial testimony, along with other defense experts, helped keep two prosecution audio experts from testifying. One prosecution expert ruled out that it was Zimmerman screaming on the 911 call and the other thought it was the teen.
Judge Debra Nelson ruled that the methods used by the experts aren't reliable. Her ruling didn't prevent the 911 calls from being played at trial.
More than 20 witnesses last week testified during the opening week of a testimony in a trial that has opened up national debates about race, equal justice, self-defense and gun control.
Zimmerman has said he fatally shot the teen in February 2012 in self-defense as the Miami-area black teenager was banging his head into the concrete sidewalk behind the townhomes in a gated community. Zimmerman is charged with second-degree murder and has pleaded not guilty.
Zimmerman defense attorney Mark O'Mara said at the end of last week that the trial was progressing at a faster pace than anticipated, but that he was reserving judgment on the prosecution's case so far.
"We're in the middle of it. They've got a lot more to show. These things build up slow, and it's sort of like pieces of a puzzle," O'Mara said. "People say, 'wait a minute, I can't see the picture yet.' They're very good prosecutors, they're gonna do very good job, and they're gonna put on their evidence. We'll see how it goes. We're certainly ready to respond to it."
Zimmerman, 29, could get life in prison if convicted of second-degree murder. The state argued during its opening statement that Zimmerman profiled and followed Martin in his truck and called a police dispatch number before he and the teen got into a fight.
Zimmerman has denied the confrontation had anything to do with race, as Martin's family and their supporters have claimed.