George Zimmerman Update: Judge denies defense request to delay trial as jury selection begins

(CBS) -- A judge on Monday denied another request by George Zimmerman defense attorney Mark O'Mara to delay the second-degree murder trial for the former neighborhood watch captain, the Orlando Sentinel reports.

PICTURES: George Zimmerman in court

READ: Trayvon Martin Shooting: A timeline of events

The denial came as jury selection launched Monday in the high-profile case. Zimmerman, 29, is accused of killing unarmed teen Trayvon Martin during an altercation in Sanford, Fla. last year. He claims he killed the teen in self-defense.

Circuit Judge Debra Nelson has denied O'Mara's repeated requests for more time to prepare for trial. Monday, O'Mara once again requested a delay, citing data from Martin's cell phone he said the defense team recently received, reports the paper.

The jury will be whittled down from a pool of about 500 candidates. The process could take up to three weeks.

Also on Monday, a hearing was expected to continue regarding whether or not certain speaker recognition specialists would be allowed to testify at trial. The judge has not yet ruled on whether the jury will hear from the experts, who have come to varying conclusions about screaming voices in the background of a 911 call placed the night Martin was killed.

One controversial state witness says he heard Martin saying "I'm begging you" in the background of the call, speech that hasn't been identified by other experts.

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