(CBS) Testifying for the defense during a pre-trial hearing Thursday, an FBI voice-recognition scientist said that he didn't believe it was possible to identify who was screaming in the background of a 911 call placed the night George Zimmerman shot and killed Florida teen Trayvon Martin.
The testimony of Hirotaka Nakasone capped a day of proceedings ahead of Zimmerman's second-degree murder trial, which launches with jury selection on Monday. Zimmerman argues he killed Martin, 17, in self-defense during an altercation in a Sanford, Fla. gated community last year.
A major decision before the judge during the proceedings, which will resume again at 9 a.m. Friday, will be whether or not to allow the testimony of a state audio expert who says he heard Martin screaming and saying "I'm begging you" in the background of the same call.
If allowed, the testimony could be key for the prosecution. But with proceedings still ongoing, the judge has yet to rule on the subject.
Experts have come to mixed conclusions about the screaming in the background of the 911 call, which was placed by a neighbor. Nakasone said that the screaming lasted for about 18 seconds - however, the amount of screaming that was not "stepped on" by the voice of the 911 dispatcher or caller was only about three seconds.
He said because the screaming voice was affected by distress or emotion, there was no way to compare it to "reasonable, natural speech and come up with correct answers."
"The screaming was not normal - it was pretty much uttered by someone who was facing probably imminent threat of death," he said. "Very difficult to analyze."
The defense has argued that the state expert who heard Martin screaming used unproven scientific techniques, and Zimmerman's brother Robert Zimmerman told CBS News' Crimesider the finding amounted to "voodoo forensics."
Also Thursday, a court information technology director testified that he found more than 1,000 additional photos -- including photos of marijuana and a hand holding a gun - and some deleted text messages on Trayvon Martin's phone. Defense attorney Mark O'Mara has maintained that prosecutors didn't turn over the information, and has asked the court to sanction prosecutors.
The judge ruled that the issue of possible sanctions should be taken up after trial, and also rejected a defense request to allow a handful of jurors to testify anonymously.