MIAMI (AP) -- The news media are taking on an increasingly police-like role in the Trayvon Martin slaying by using modern forensic techniques to analyze evidence, an approach some legal experts say can lead to a distorted view of the case because a lot of the key evidence is still under wraps.

The public has been whipsawed back and forth as new revelations emerge, appearing to support one version or the other.

Most recently, the Orlando Sentinel had a voice analysis expert examine a 911 call in which a person is heard screaming for help before the fatal gunshot. The shooter, neighborhood watch volunteer George Zimmerman, told police he was yelling, but the expert told the newspaper the voice most likely was not Zimmerman.

"It would be nice to know who was doing the calling for help, but identifying the caller is not necessarily going to definitively identify the wrongdoer," said University of Florida law professor Bob Dekle, a former state prosecutor and public defender. "Situations sometimes arise where it is the wrongdoer calling for help."

ABC News on Monday aired what it said was an enhanced version of a police video taken the night of the shooting that appeared to show wounds or welts on the back of Zimmerman's head. The initial, grainier video aired last week seemed to show no wounds or blood, which led Martin's family and supporters to proclaim that it undercut Zimmerman's story.

Legal and forensic experts cautioned that none of the media-led investigations, which are done in many high-profile cases, has been conclusive.

"The public needs to know that this is a very complex case," said Ron Martinelli, a forensics consultant in Temecula, Calif. "There are many issues that come into play and sometimes come into conflict."

Zimmerman told police that he was attacked by Martin on Feb. 26 and believed he had no choice but to fire his gun at Martin in self-defense. The teenager's family believes Zimmerman, 28, singled Martin out as suspicious because he was black. Zimmerman's father is white and his mother is Hispanic.

The family also said Zimmerman should've listened to a police dispatcher who told him not to follow Martin.

The 911 call analysis compared the screams of "Help" to other recordings of Zimmerman's voice using a computerized matching technique. The consultant, Tom Owen, told the Sentinel that the comparison showed a 48 percent match between the two samples. A positive match should be above 90 percent, he said. Owen did not respond to an email Monday seeking additional comment.

If Zimmerman is charged and the case goes to court, the defense would likely hire experts to punch holes in any conclusions about the 911 tape or the police video.

"The other side will have experts saying `you can't make anything out of this, it's all garbled, look how much they had to enhance it,'" said Robert Jarvis, a law professor at Nova Southeastern University. "What has been done with the tape? Has anybody tampered with it?"

News organizations also used audio technology to enhance a 911 call in which some reported that Zimmerman muttered a racial epithet under his breath. Other media organizations, including The Associated Press, said the raw recording was not clear enough to determine what Zimmerman actually said.

Experts have also said that photos initially released of Martin and Zimmerman could have skewed initial public perceptions.

Most of the photos of Martin show a baby-faced boy in a red T-shirt or football uniform, much younger than the tall teenager he was the night of the shooting. And Zimmerman no longer resembles the beefy-looking figure pictured in a mug shot from several years ago.

Special prosecutor Angela Corey has been appointed by Gov. Rick Scott to lead the investigation after Sanford officials declined to make an arrest. The Justice Department and FBI are also investigating.

Much still is not known about the evidence being weighed by prosecutors, including:

-The autopsy on Martin's body, which could show signs of a fight and whether the bullet entry wound supports Zimmerman's claims.

-Medical records of treatment Zimmerman received on the scene that night by paramedics, which again could back up or disprove his self-defense assertion.

-Police photographs, notes and other physical evidence probably collected at the scene.

-Videotaped interviews police conducted with Zimmerman.

-Whether there is other surveillance video of Martin or Zimmerman at any point during the evening, including the youth's visit to a convenience store shortly before the confrontation.

-Whether any witnesses saw the actual shooting and the circumstances leading up to it, including the alleged fight.

Many of these unanswered questions, especially the forensic results, will provide a more complete picture of how Zimmerman came to shoot Martin that night, Martinelli said.

"The decedent gets to have a voice only through forensics," he said. "That's how people speak from the dead."

---

Follow Curt Anderson on Twitter: http://twitter.com/Miamicurt

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Learn more about our Privacy Policy and Terms of Use.