SANFORD, Florida (CNN) -- Outrage over the killing of an unarmed Florida teen rippled nationwide as supporters planned more protests Wednesday and a petition demanding the shooter's arrest amassed nearly 1 million signatures.
Trayvon Martin was fatally shot February 26 while walking to the house of his father's fiancee in Sanford after a trip to a convenience store.
George Zimmerman, a neighborhood watch leader, said he shot the teen in self-defense.
Zimmerman has not been arrested or charged in the killing of the black teenager. A police report describes Zimmerman as a white male, but his family says he is Hispanic.
Demonstrators demanded justice at a Tuesday night rally at a Sanford church, where Ben Jealous, the head of the NAACP, called for the police Chief Bill Lee's resignation.
Jealous said Lee's department mishandled the case by not arresting Zimmerman.
The U.S. Justice Department has also launched a civil rights investigation into the shooting. Jealous said the federal agency's decision to get involved has empowered Trayvon's supporters to stand their ground.
Nearly 750,000 people have signed a petition on Change.org demanding Zimmerman's arrest, making it one of the website's largest campaigns.
On Wednesday, supporters will gather in New York City for a "Million Hoodie March," a reference to the attire the 17-year-old was wearing when he was shot.
"A black person in a hoodie isn't automatically suspicious. Let's put an end to racial profiling," the protest page said.
The February 26 shooting occurred when Zimmerman -- who was patrolling the neighborhood -- saw the teen walking home after buying candy and a drink at a convenience store.
Zimmerman called 911 and reported what he described as a suspicious person. A few moments later, several neighbors called the emergency number to report a commotion outside.
Heated debate erupted over whether Zimmerman used a racial slur during the 911 call released this week.
"We didn't hear it, however, I am not sure what was said. So I never said we missed a racist remark," said Sgt. David Morgenstern of the Sanford Police Department.
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 that Zimmerman used a racial slur.
Benjamin Crump, the Martin family's attorney, said the 911 call was questionable even without the slur.
"Without even hearing the conflicting part, we did hear number one that he said 'these people,'" Crump told CNN. "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.
While some neighbors were still on the phone with the emergency dispatchers, cries for help followed by a gunshot sounded in the background.
"The time that we heard the whining and then the gunshot, we did not hear any wrestling, no punching, no fighting, nothing to make it sound like there was a fight," said Mary Cutcher, one of the callers.
Cutcher said Zimmerman was confused after the shooting.
"He'd pace and go back to the body and just like -- I don't know if he was kind of 'Oh, my God, what did I do? what happened?' " she said.
Another caller, Selma Mora Lamilla, said she did not hear any altercation, but the teen cried and "whimpered" before the shooting.
She described Zimmerman as "straddling" the teen after the shooting, saying he was "on his knees on top of a body."
Crump said Martin's girlfriend was on the phone with him during the incident and can help prove he was killed "in cold blood."
The girl connects the dots and "completely blows Zimmerman's absurd self-defense claim out of the water," Crump told reporters Tuesday.
Shortly before he was shot, the teen told his girlfriend that someone was following him, and he was trying to get away, according to the lawyer. The girl, who did not want to be identified, said that during the call, she heard the teen ask why the person was following him.
She got the impression there was an altercation in which his earpiece fell out after he was pushed and the connection went dead, Crump said.
She did not hear any gunfire, he said.
Phone records show the teen was on the phone with her much of the day, including around the time of the killing, the lawyer said.
A Seminole County grand jury will convene April 10 on the matter, State Attorney Norm Wolfinger said in a statement.
Martin's family said they believe race was a factor in his death, fueling an outcry in the racially mixed community 16 miles northeast of Orlando.
Zimmerman's family has denied race played a role, saying he has many minority relatives and friends.
CNN has made numerous attempts to contact
Zimmerman, but has been unsuccessful. His father told a Florida newspaper his son had moved after receiving death threats.
Police say they have not charged Zimmerman, 28, because they have no evidence to contradict his story that he shot in self-defense.
In a police report, Officer Timothy Smith said Zimmerman stated 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 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 911 recordings tell a different story.
During the incident, the teen started to run, Zimmerman reported.
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 Jeffrey Toobin, a senior legal analyst for CNN.
"Clearly, the question at the heart of the case is whether Zimmerman reasonably felt threatened. On this issue, the evidence currently seems murky," Toobin said.
Finding other witnesses is crucial because the teen is no longer here to 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."
-- CNN's Vivian Kuo, David Mattingly, Brian Vitagliano and Moni Basu contributed to this report.
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