Who screams on 911 call in Trayvon Martin case?

(CNN) -- Screams recorded on a 911 call during the confrontation that ended in Florida teenager Trayvon Martin's death don't seem to be those of neighborhood watch volunteer George Zimmerman, two audio experts who analyzed the recordings said Monday.

Zimmerman, 28, has claimed self-defense in shooting Martin on February 26, saying the 17-year-old accosted him after he had called police to report the teenager as a suspicious person walking around his neighborhood.

But audio experts Tom Owen and Ed Primeau, who analyzed the recordings for the Orlando Sentinel using different techniques, said they don't believe it's Zimmerman who can be heard screaming in the background of the 911 calls.

"There's a huge chance that this is not Zimmerman's voice," said Primeau, a longtime audio engineer who is listed as an expert in recorded evidence by the American College of Forensic Examiners International. "As a matter of fact, after 28 years of doing this, I would put my reputation on the line and say this is not George Zimmerman screaming."

Owen, a forensic audio analyst who is also chairman emeritus of the American Board of Recorded Evidence, said he also does not believe the screams come from Zimmerman.

Software frequently used to analyze voices in legal cases shows a 48% likelihood that the voice is Zimmerman's. At least 60% is necessary to feel confident that two samples are from the same source, he said Monday on CNN. That means it's unlikely the screams came from Zimmerman, Owen said.

The experts, both of whom say they have testified in cases involving audio analysis, stressed they cannot say who was screaming. They have no samples of Martin's voice.

Zimmerman has told police that he was screaming for help.

The revelations come as the special prosecutor in the case continues her investigation into the shooting, which has fueled protests nationwide, sparked calls for Zimmerman's arrest, renewed attention to race relations and caused discussion of the repeal of Florida's "stand your ground" law.

Sanford police have cited the law as an explanation for why Zimmerman has not been arrested. The law allows people to use deadly force anywhere they feel a reasonable threat of death or serious injury.

According to an Orlando Sentinel account later verified by police, Zimmerman has said Martin punched him in the face, knocked him to the ground and slammed his head on the concrete before the shooting.

Zimmerman's brother has said medical records would prove his brother's account, but Martin's family and supporters have rejected that version.

They say Zimmerman, who is Hispanic, racially profiled the teen, who was black, and ignored a police dispatcher's directive not to follow him.

Protesters have also called for the firing of Sanford police Chief Bill Lee, who stepped aside in March amid mounting criticism of his department's handling of the case.

On Monday, Martin's parents are expected to ask the U.S. Justice Department to review the early actions of State Attorney Norm Wolfinger, the prosecutor for Brevard and Seminole counties, who stepped aside early in the investigation.

The family will ask Justice Department investigators to look into possible interference by Wolfinger immediately after the shooting. Wolfinger is no longer involved in the case.

ABC News has reported that the lead homicide investigator, Chris Serino, filed an affidavit pushing for charges the night of the killing, but was overruled by the state attorney's office.

"It certainly confirmed all of my thoughts that this investigation had been botched from the beginning and that people other than me knew that there was supposed to be an arrest made," Martin's father, Tracy, told CNN after the ABC News report.

Meanwhile, a gun-rights group called legalboom.com has begun raising funds for a legal defense fund for Zimmerman.

Christopher Kossman, who is organizing the campaign, declined to say how much money the effort has raised, but said it has been "overwhelming."

"If it needs to go to a trial for justice to be served, then so be it," Kossman said. "But judging him in the court of public opinion is not the way to go about this."

CNN's Anderson Cooper and Vivian Kuo contributed to this report.