FILE - George Zimmerman, right, and attorney Don West, stand as the judge enters the courtroom in Seminole circuit court for a pretrial hearing, in Sanford, Fla., in this Saturday, June 8, 2013 file photo. Jury selection begins Monday June 10, 2013 in the second-degree murder trial, which is expected to last about six weeks. Martin's killing drew worldwide attention as it sparked a national debate about race, equal justice under the law and gun control. (AP Photo/Orlando Sentinel, Joe Burbank, Pool)
SANFORD, Fla. (AP) -- An FBI audio expert whose pre-trial testimony helped keep prosecution witnesses from testifying at George Zimmerman's murder trial has been called to the witness stand.
Hirotaka Nakasone started testifying Monday to begin the second week of testimony in Zimmerman's second-degree murder trial.
Zimmerman is pleading not guilty to second-degree murder. He says he acted in self-defense when he fatally shot 17-year-old Trayvon Martin last year.
During a pre-trial hearing, Nakasone testified that there wasn't enough clear sound to determine whether Zimmerman or Martin was screaming in 911 calls.
Based on that, Judge Debra Nelson decided not to allow two prosecution witnesses to testify. One ruled out that it was Zimmerman screaming and the other thought it was the teen.