911 tape analysis: Is George Zimmerman's story falling apart?

Only one person alive knows exactly what happened in the final confrontation between 17-year-old Trayvon Martin and George Zimmerman, the 28-year-old neighborhood watch volunteer who shot Martin dead on Feb. 26. That man, of course, is Zimmerman, and he reportedly told police that after calling 911 to alert the cops to Martin's "suspicious" behavior, Martin attacked him and bloodied him up, spurring Zimmerman to shoot Martin in self-defense. As a special prosecutor looks into whether Zimmerman's story holds up to scrutiny, the news media is finding some evidence that it may not. The latest: Forensic voice identification experts tell the Orlando Sentinel that the final screams before the fatal gunshot — captured in a neighbor's 911 call — aren't from Zimmerman, as his supporters claim. How do the experts know, and how damning is this clue? Here's what you should know:

How was this analysis conducted?

The Sentinel spoke with two experts. The first, biometric voice analyst Tom Owen, who studies the characteristics that make each voice unique and identifiable. After using sophisticated computer technology to compare the scream in the background of the neighbor's 911 calls to Zimmerman's own 911 calls, Owen tells the Sentinel that "you can say with reasonable scientific certainty that it's not Zimmerman" screaming. The second expert, Ed Primeau, enhanced the audio recordings and relied on his ears — and years of forensic audio experience. Based on tone, "I believe that's Trayvon Martin in the background, without a doubt," he says. "That's a young man screaming."

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How credible is this evidence?

Owen is chairman emeritus for the American Board of Recorded Evidence, and his analysis in a recently decided Connecticut murder case helped seal defendant Sheila Davalloo's conviction. The Zimmerman audio is much cleaner than Davalloo's — a "home run," Owen says — and he tells MSNBC that he believes his analysis would be admissible in court. Audio enhancement expert Arlo West tells the Sentinel that thanks to TV shows like CSI, juries place a lot of faith in forensic evidence, as long as it seems conclusive.

Are there weaknesses in the forensic audio case?

The biggest missing piece is that Owen doesn't prove that the screams came from Martin — he doesn't have any recordings of Martin's voice to compare. Conservative blogger Tom Maguire at Just One Minute suggests that Zimmerman's lawyers would seize on this shortcoming and "uncharitably point out that it might not be possible to match the voice to either person," then wait for the judge to "expound on 'innocent until proven guilty' and 'reasonable doubt.'"

Is Zimmerman's case falling apart?

Following last week's release of police video showing an apparently unharmed Zimmerman soon after Martin allegedly beat his head on the ground, "the pile of evidence bringing Zimmerman's claims into question grows deeper every day," says Timothy Lange at Daily Kos. And the question of who is screaming before the gunshot cuts to "the key issue in the case": Was it self-defense? This voice analysis "could spell big trouble for George Zimmerman," agrees Dan Abrams at Mediaite, especially if it's confirmed by other voice ID experts.

Is the media out to get Zimmerman?

Not necessarily, though trial-by-media does have its risks, says Erik Wemple at The Washington Post. For instance, last week, the Today show ran an unfairly edited version of Zimmerman's 911 call: "This guy looks like he's up to no good," Zimmerman said. "He looks black." The problem? Today cut out the part where the operator asked Zimmerman about Martin's race. It could be worse, says Eric Zorn in the Chicago Tribune. Imagine if Zimmerman were black and Martin white, and "Fox News were virtually operating a non-stop crusade... to get the black guy arrested."

Sources: Chicago Tribune, Daily Kos, Just One Minute, Mediaite, MSNBC, Orlando Sentinel, Washington Post