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(CNN) -- An "outraged" Florida prosecutor fired back on Monday at the family of Trayvon Martin, describing as "outright lies" their account that he and a local police chief met and decided not to follow a detective's advice and arrest the teenager's killer.
Earlier in the day, the Martin family delivered a letter to the U.S. Justice Department through attorney Benjamin Crump's office requesting a federal investigation of the decision not to arrest George Zimmerman after the fatal February 26 shooting.
The 28-year-old neighborhood watch volunteer has said he killed Martin in self-defense, saying the teen punched him and slammed his head into a sidewalk before the shooting, according to family members and police.
Martin's family and supporters say Zimmerman, who is Hispanic, profiled the boy, who is black, as "suspicious" and ignored a police dispatcher's request that he not follow him. The 17-year-old had a bag of Skittles candy and an iced tea at the time of his death.
Martin case spotlights racial profiling
In the letter delivered Monday, the Martin family said that a Sanford police detective "filed an affidavit stating that he did not find Zimmerman's statements credible in light of the circumstances and facts surrounding the shooting."
The family said Sanford Police Chief Bill Lee and State Attorney Norm Wolfinger met the night of the shooting and disregarded the detective's advice, letting Zimmerman go.
Neither Sanford police nor prosecutors have confirmed the existence of such an affidavit, which ABC News first reported. Sanford officials and special prosecutor Angela Corey's office declined comment.
But Wolfinger, who stepped aside in the case last month, vehemently denies that any "such meeting or communication occurred" between him and Lee.
Video: Case divides Florida town
"I have been encouraging those spreading the irresponsible rhetoric to stop and allow State Attorney Angela Corey to complete her work," Wolfinger said in a statement Monday. "Another falsehood distributed to the media does nothing to forward that process."
Sanford police have cited Florida's "stand your ground" 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.
The Martin case has triggered a nationwide debate about that law and race in America, as well as generated criticism into how authorities have handled the matter. In March, Lee stepped down temporarily as Sanford's police chief -- though some activists are still calling for his firing, in addition to Zimmerman's arrest.
Much talk Monday centered on one potentially critical piece of evidence: Who is the person heard screaming "help" in a 911 call just before the shooting?
What witnesses say
Zimmerman -- according to his family members and his account to authorities, as first reported by the Orlando Sentinel and later confirmed by Sanford police -- said he was yelling for help. The shooting victim's family, including his cousin Ronquavis Fulton and others, meanwhile, have said they are sure the voice heard on the 911 call is that of Trayvon Martin.
Audio experts Tom Owen and Ed Primeau, who analyzed the recordings for the Sentinel using different techniques, said they don't believe it is Zimmerman who is heard yelling in the background of one 911 call. They compared those screams with Zimmerman's voice, as recorded in a 911 call he made minutes earlier describing a "suspicious" black male, who ended up being Martin.
"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. "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 and chairman emeritus of the American Board of Recorded Evidence, also said he does not believe the screams came from Zimmerman.
He cited software frequently used to analyze voices in legal cases, by investigating characteristics such as pitch and the space between spoken words, that found a 48% likelihood the voice is Zimmerman's. At least 60% is necessary to feel confident two samples are from the same source, he told CNN on Monday -- meaning it's unlikely it was Zimmerman who can be heard yelling.
Opinion: A rush to judgment?
The experts, both of whom said they have testified in cases involving audio analysis, stressed they cannot say who was screaming. They have no samples of Martin's voice.
Media, race and polarization
Such analysis could play a role should there be a criminal or civil case over Martin's death. Primeau, who said he uses a combination of critical listening skills and spectrum analysis, called voice identification "an exact science" that can help a legal team in court.
Yet standards set by the American Board of Recorded Evidence indicate "there must be at least 10 comparable words between two voice samples to reach a minimal decision criteria." While Zimmerman says more than that many words on his 911 call, the only one heard on the second is a cry for "help."
David Faigman, a professor of law at the University of California-Hastings and an expert on the admissibility of scientific evidence, said courts and the overall scientific community have mixed opinions about the reliability of such "voiceprint" analysis.
Because one goal in the Martin case might be ruling out Zimmerman as the source of the screams, rather than precisely identifying who actually was yelling, it could lower the bar for getting such evidence into court, he said.
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Still, he said, it wouldn't be too hard for Zimmerman's attorneys to find an audio expert to offer an opposing opinion.
"These expert witnesses come out of the woodwork when money is concerned," he said.
The next major movement in the case likely will come from Corey's office, which could charge Zimmerman, clear him or send the case to a grand jury.
Meanwhile, the debate continues to rage about the case. Civil rights activists, like the Rev. Al Sharpton and the Rev. Jesse Jackson, have partnered with the Martin family in recent weeks in several high-profile, well-attended rallies calling for "justice" and Zimmerman's arrest in the case.
Zimmerman himself is getting support from, among others, a gun rights group called legalboom.com that 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.