"I have listened to the tapes, and I have not heard them use a racial slur," concurred Sanford City Manager Norton Bonaparte.

A top CNN audio engineer enhanced the sound of the 911 call, and several members of CNN's editorial staff repeatedly reviewed the tape but could reach no consensus on whether Zimmerman used a racial slur.

Whether Zimmerman used such language prior to shooting Martin is key, according to CNN Senior Legal Analyst Jeffrey Toobin. "It's extremely, extremely significant because the federal government is not allowed to prosecute just your ordinary, everyday murder. Two people fighting on the street is not a federal crime. However, if one person shoots another based on racial hostility, racial animus, that does become a federal crime."

Toobin said that if "very shortly before" the shooting, "Zimmerman used this racial epithet to refer to the person he openly shot, that very much puts it within the FBI's and the Justice Department's ambit of a case that they could prosecute."

The Martin family's lawyer said the 911 call was questionable even if there was no slur.

"Without even hearing the conflicting part, we did hear, number one, that he said 'these people,'" said Crump"What did he mean when he said 'these people'? He also profiled him because he was a young black person with a hood on.

"So it goes without saying, even if you don't get to the thing that everyone is debating, he already had a mentality when he got out of that car that this was a young black man, and he had assumed that he was a criminal, and you know what happens when you assume," Crump said.

Police say they have not charged Zimmerman because they have no evidence to contradict his story that he shot in self-defense.

In a police report, Officer Timothy Smith said Zimmerman said he was "yelling for someone to help me," but the victim's family said it was the teen asking for help.

The shooting has renewed a debate over a controversial state law and sparked calls for a review.

Florida's deadly force law, also called "stand your ground," allows people to meet "force with force" if they believe that they or someone else is in danger of being seriously harmed by an assailant, but exactly what happened in the moments leading up to Martin's death remains unclear.

Zimmerman's father said his son never followed or confronted the teen, but the 911 recordings tell a different story.

During the incident, the teen started to run, Zimmerman said.

When Zimmerman said he was following the teenager, the dispatcher told him, "We don't need you to do that."

The case is rooted in one main thing, said Toobin: "Clearly, the question at the heart of the case is whether Zimmerman reasonably felt threatened. On this issue, the evidence currently seems murky."

Finding other witnesses is crucial because the teen cannot give his side, he said.

State Sen. Oscar Braynon II sent a letter to Florida Senate President Mike Haridopolos asking for a review of the stand-your-ground law. Braynon called for a legislative panel to look into how the law has been used and implemented.

"The ultimate goal of such process is to decrease the number of incidents like that of Trayvon's and discourage more individuals from deciding to become vigilantes resulting in more lives lost," Braynon wrote.

Florida Gov. Rick Scott said he was going to look into the law "because if what's happening is that it's being abused, that's not right."