The shooting death in Florida last month of 17-year-old Trayvon Martin has sparked a national outcry demanding that the man who shot the unarmed black youth be charged in his death. As of Tuesday morning, more than 500,000 people had signed an online petition at Moveon.org seeking the prosecution of George Zimmerman, 28, the neighborhood watch volunteer who claims he shot the young man in self-defense.

On Monday, three weeks after the incident, the Justice Department announced that its Civil Rights Division, in conjunction with the Federal Bureau of Investigation, would investigate the February 26 shooting death. The Justice Department also said it would provide assistance to state officials in their investigation into the incident. The state attorney’s office for Florida’s Brevard and Seminole counties announced Tuesday that a grand jury will hear evidence April 10 into the events surrounding Trayvon’s death.

The incident has focused attention on Florida’s “Stand Your Ground” law, which allows alleged potential crime victims “in fear of great bodily injury” to use deadly force in public places. Critics of the law, passed in 2005 under Governor Jeb Bush, have dubbed it the “Shoot first, ask questions later” law.

The fatal shooting took place in a predominantly white, gated community in Sanford, Florida, about 20 miles north of Orlando. Following three weeks of pressure from the slain boy’s family and their supporters, the Sanford Police Department released the 911 calls received by dispatchers leading up to and following the shooting. The contents of the calls were recounted to ABC News affiliate WFTV by a representative of the boy’s family, Ryan Julison.

Trayvon Martin, who lived in Miami with his mother, was visiting a friend of his father’s in Sanford on the rainy evening in question, and had gone to the local convenience store to purchase some snacks. Zimmerman, who was patrolling the neighborhood in his sport utility vehicle, spotted Martin shortly after 7 p.m. wearing a sweatshirt with the hood up.

According to Julison, in Zimmerman’s first 911 call to a non-emergency dispatch number, he says, referring to Trayvon, “He’s checking me out,” followed by, “This guy looks like he’s on drugs, he’s definitely messed up.” He adds, “These a**holes always get away.”

In another call, Zimmerman says, “He’s here now just looking at all the houses. Now he’s just staring at me,” adding a few seconds later, “He’s coming to check me out, he’s got something in his hands.”

Zimmerman then reportedly got out of his car, carrying his licensed 9-millimeter pistol. The dispatcher is heard asking, “Are you following him?... Okay, we don’t need you to do that.” Trayvon began to run at this point, followed by Zimmerman, despite the dispatcher’s instructions to Zimmerman to wait for the police.

Neighbors then began to call 911. According to witnesses who spoke to ABC News, a fight broke out between the two. At one point Zimmerman, who outweighed the boy by 100 pounds, was reportedly on the ground with Trayvon on top. In one call, a cry for help and terrified howls are heard in the background. One man tells the dispatcher, “There is two guys in the backyard with flashlights. There is a black guy down and it looks like he’s been shot and he’s dead.”

It is apparently not clear from the 911 audio whether one or two shots were fired, and who is crying out for help. Trayvon’s mother, Sybrina Fulton, commented to Reuters after listening to the tapes, “I recognized that [voice] as my baby screaming for help before his life was taken.”

When police arrived, Trayvon was found face down with a fatal bullet wound to the chest. He was carrying candy, a can of iced tea, and some cash. Only one shell casing, from Zimmerman’s weapon, was found at the scene.

Tracy Martin, Trayvon’s father, was quoted in the New York Times as saying: “There is no justice in this. The public is outraged because my son only had snacks in his pocket, no weapon whatsoever, not even a fingernail file. For him to be murdered by someone who weighs more than 100 pounds more than him, more than 10 years older than him, this is an outrage.”

Zimmerman claims he acted in self-defense. The Sanford police have accepted his version of events and no charges have been filed against him. He was not tested for drugs or alcohol in relation to the incident, although this is standard procedure in most homicide investigations.

Detectives told the slain boy’s parents that Zimmerman had a “squeaky clean” record. But when a criminal background check was finally run the next day, it was found that the shooter had been arrested in 2005 on charges of “battery on a law enforcement officer,” charges that were later dropped.

One witness to the shooting, Mary Cutcher, said she did not believe it was in self-defense. She added that the police took only a brief statement from her despite her efforts to go into detail. ABC News also reported that one of the responding officers “corrected a witness after she told him that she heard the teen cry for help.”

The officer in charge of the scene of Trayvon’s shooting was involved in another case that drew attention in 2010, in which a police lieutenant’s son was videotaped attacking a black homeless man. The same officer who has not arrested Zimmerman failed initially to arrest the police lieutenant’s son in the earlier case. The attacker was later arrested when local channel WFTV broke the news of the incident.

The promotion of law-and-order by the political establishment and media has fostered an environment where armed “citizens watch” groups are given wide latitude by authorities to patrol gated and other communities in pursuit of supposedly suspicious individuals. In Florida and more than a dozen other states, these vigilante-type patrols are emboldened by laws that legalize the use of weapons under conditions that are not self-defense.

Brian Tannebaum, a criminal defense lawyer in Florida, told the Times, “Stand Your Ground is a law that has really created a Wild West type environment in Florida. It allows people to kill people outside of their homes, if they are in reasonable fear for their lives. It’s a very low standard.”