The release of evidence did little to clear up whose voice is screaming for help in the background of several 911 calls made during the fight.

Since first hearing the calls in early March, Martin's mother, Sybrina Fulton, has been unequivocal in saying it was her son's voice on the tapes.

But Serino wrote in a report that he played a 911 call for Martin's father, Tracy, in which the screams are heard multiple times.

"I asked Mr. Martin if the voice calling for help was that of his son," the officer wrote. "Mr. Martin, clearly emotionally impacted by the recording, quietly responded `no.'"

Zimmerman's father also told investigators that it was his son yelling for help on March 19.

"That is absolutely positively George Zimmerman," Robert Zimmerman said. "He was not just yelling, he sounded like he was screaming for his life."

Investigators sent all the recordings to the FBI for analysis. They were asked to determine who was screaming, and also if Zimmerman might have used an expletive in describing Martin. Prosecutors said in their charging documents that Zimmerman said "(expletive) punks" in describing Martin as he walked in the neighborhood.

But the analyst who examined the recordings determined the sound quality is too poor to decipher what Zimmerman uttered. In regards to the screams during the altercation, there also wasn't enough clarity to determine who it is "due to extreme stress and unsuitable audio quality."

The case has become a national racial flashpoint because the Martin family and supporters contend Zimmerman singled Martin out because he was black. Zimmerman has a Peruvian mother and a white father.

Two acquaintances painted an unflattering picture of Zimmerman in police interviews.

A distraught woman told an investigator that she stays away from Zimmerman because he's racist and because of things he's done to her in the past, but she didn't elaborate on what happened between them.

"I don't at all know who this kid was or anything else. But I know George, and I know that he does not like black people. He would start something. He's very confrontational. It's in his blood. We'll just say that," the unidentified woman says in an audio recording.

A man whose name was deleted from the audio told investigators said he worked with Zimmerman in 2008 for a few months. It wasn't clear which company it was.

The man, who described his heritage as "Middle Eastern," said that when he first started, many employees didn't like him. Zimmerman seized on this, the employee said, and bullied him.

Zimmerman wanted to "get in" with the clique at work so he exaggerated a Middle Eastern accent when talking about the employee, the man said. The employee told investigators that Zimmerman made reference to terrorists and bombings when talking about him.

"It was so immature," said the employee, who ended up writing a letter to management about Zimmerman.

Zimmerman's parents say he wasn't racist. They say he had mentored black students and had a black relative.

The autopsy says medical examiners found THC, the psychoactive ingredient in marijuana, when they tested Martin's blood and urine.

Kobilinsky said the amount was so low that it may have been ingested days earlier and played no role in Martin's behavior. He doubts the judge will even let it be used by the defense if they try to introduce it at trial.

A police report shows the 17-year-old had been shot once in the chest and had been pronounced dead at the scene. The autopsy says the fatal shot was fired from no more than 18 inches away.