On Feb. 26, Trayvon Martin, an unarmed black teenager in Sanford, Fla., was shot dead by George Zimmerman, a 28-year-old white Hispanic man driving around his gated community in an SUV as part of the volunteer neighborhood watch program.

Since the shooting three weeks ago, Martin's family has called on the U.S. Justice Department to investigate the case and has accused both the shooter and local police, who have yet to arrest Zimmerman, of racial bias.

As 911 tapes and testimony from Martin's friend, who was talking to him on the phone moments before he was shot, have been released over the past week, the Justice Department said Tuesday that it will investigate the shooting.

Many questions, however, remain.

What happened the rainy night in February when Martin was shot in the chest shortly after he left a store armed with only an Arizona Iced Tea and a bag of Skittles? Why did Zimmerman, who called 911, get out of his car, ignoring the emergency dispatcher's instructions to stay put until police arrived? Perhaps more importantly, why are authorities claiming the shooter is protected under Florida's self-defense laws, after he tracked the 17-year-old for several minutes and then shot him to death?

From why the shooter remains a free man to what Martin told his friend moments before he shot, here are seven things to know about George Zimmerman and the Trayvon Martin Florida shooting case.

1. Feb. 26: What's Known About The Shooting

On that fateful night, neighborhood watch volunteer George Zimmerman, 28, was patrolling his gated community in Sanford, a suburb of Orlando, in his SUV, something he asserted was a nightly routine after a series of break-ins in the area.

Trayvon Martin, 17, was heading back to his father's house after running to a nearby 7-Eleven during halftime of the NBA All-Star Game on television. He was carrying Skittles, some money and a can of Arizona Iced Tea. Zimmerman was carrying a gun.

It was dark and raining when Zimmerman first spotted the black teenager, wearing a hooded sweatshirt, and called a non-emergency dispatcher.

Hey, we've had some break-ins in my neighborhood, and there's a real suspicious guy at Retreat View Circle. This guy looks like he's up to no good or on drugs or something, Zimmerman told the dispatcher in his first call. The 911 tapes from the incident were released by the Sanford police department this week.

It's raining and he's just walking around, looking about, Zimmerman, who is white and Hispanic, continued. He's here now just looking at all the houses. Now he's just staring at me. ... He's coming to check me out. He's got something in his hands. I don't know what his deal is. Can you get an officer over here?

At this point, Martin began to cut between homes in the neighborhood, running in the direction of his father's house.

Zimmerman decided to follow him.

These a--holes, they always get away, Zimmerman can be heard telling the dispatcher.

Are you following him? the dispatcher asked. Yeah, Zimmerman replied.

We don't need you to do that, the dispatcher said, advising Zimmerman to wait by a nearby mailbox for local police to arrive and evaluate the threat.

Zimmerman agreed, then changed his mind. Actually, could you have him call me and I'll tell him where I'm at? he said. Then he got out of his vehicle, holding a 9-millimeter handgun.

Calls to 911 from frightened neighbors indicate a struggle between the two ensued. A faint voice in the background of these calls can be heard crying for help. A gunshot is heard.

When police arrived on the scene, they found Martin dying of a gunshot wound to the chest. Zimmerman told police the two had gotten into a physical altercation, and he had shot the teen in self-defense.

2. Racism, Self-Defense: Zimmerman's Cloudy Motivations

When police arrived on the scene, George Zimmerman dropped the gun and told them he had shot Trayvon Martin in self-defense.

Sanford officers believed him, declining to charge him and telling Martin's family that Zimmerman had a squeaky clean record.

Yet, Zimmerman had been arrested in 2005 on charges of resisting arrest with violence and battery on an officer, though these charges were later dropped. Police also failed to give him a drug or alcohol test, part of standard police procedure.

Officers claim that they had to make do with the information they were given at the time, and that they performed a criminal background check as fast as they could.

But to Martin's family and other local residents, that Zimmerman hasn't been charged with murder more than three weeks following the shooting an unarmed teen smacks of injustice, not procedure.

The circumstances of the case, meanwhile, appear to Martin's family to indicate that Zimmerman targeted their son while patrolling because he was black.

The Miami Herald reported Zimmerman called police 46 times between Jan. 1, 2011, and Feb. 26, 2012, the night of the shooting, with alerts while on patrol. Almost all those calls involved sightings of black males within the mostly white, gated community.

He was reacting to the color of his skin, Martin's mother, Sybrina Fulton, said. He committed no crime.

Everybody is outraged, Tracy Martin, Trayvon's father, added. 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, Martin told the New York Times. For him to be murdered by someone who weighs 100 pounds more than him, more than 10 years older than him, this is an outrage.

Recent claims by witnesses that their police interviews were cursory (a charge the Sanford police department vehemently denies) fuel speculation Zimmerman is being shielded from justice in an area with a history of racial tension.

In 2011, former Sanford Police Chief Brian Tooley resigned after a YouTube video showed his 21-year-old son attacking a homeless black man. In 2005, two white security guards, one of them also the son of a Sanford police officer, shot and killed 16-year-old Travares McGill.

Benjamin Crump, the Martin family's lawyer, also believes Zimmerman's attack was racially motivated, and officers' failure to arrest a man who 911 tapes reveal stalked and shot an unarmed 17-year-old is nothing short of a gross miscarriage of justice.

Had Trayvon been the person who was the triggerman, they would have arrested him from Day 1 and they wouldn't have given him bail and he would be sitting in jail, Crump told the Times. Zimmerman is free and sleeping in his own bed at night.

Chief Bill Lee of the Sanford police takes issue with these claims, especially where police procedure is concerned.

Our investigation is color blind and based on the facts and circumstances, not color, he told the New York Times. I know I can say that until I am blue in the face, but, as a white man in a uniform, I know it doesn't mean anything to anybody.

Zimmerman's father, meanwhile, delivered a one-page letter to the Orlando Sentinel on Thursday asserting his son grew up in a multiracial family and would be the last to discriminate for any reason whatsoever.

Sanford's black community remains unconvinced. One of the witnesses in the Florida shooting, a 13-year-old black boy who shall remain anonymous, recorded a video for the Orlando Sentinel detailing what he heard and saw.

I just think that sometimes people get stereotyped, he said. And I fit into the same stereotype as the person who got shot.

3. Confusion at Crime Scene: Who Called For Help?

George Zimmerman may have made the first 911 calls of the night to the dispatchers on call on Feb. 26, but he was far from the last.

Minutes after his initial conversation, lines were swamped with calls from anxious neighbors, who heard cries for help and terrified howls in the dark as two men struggled just outside.

A guy yelled: 'Help! Oh, my God,' one man told 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.

Another caller said: I think they're yelling 'Help!' but I don't know. Send someone quickly, please.

Audio recordings of the 911 tape don't make it clear whether one or two shots, with the first possibly a warning, were fired. More crucial to Martin's parents and Zimmerman's story, however, is the question of which of the two men was calling for assistance.

The police told the Sentinel they believed that the voice crying for help was Zimmerman's, a claim Zimmerman himself has asserted. Some of those who knew the shooter were quick to describe him as a conscientious volunteer, one who had prevented burglaries in the past and would likely have called for help if he needed it.

Many of the witnesses involved in the case, however, believe the voice to be Martin's, as do the young man's parents, who pointed out Zimmerman weighs 100 pounds more than their teenage son.

Tracey Martin and Sybrina Fulton say they have no doubt that it is their son pleading for his life on the 911 tape, not Zimmerman asking for help.

I listened to the tapes and it just broke my heart again to hear him screaming out for help and pleading for his life, and he was still murdered, his mother told the Times. There is no question in my mind that is his voice.

4. Trayvon Martin's Final Moments

This latter belief may be supported by testimony from one of Trayvon Martin's friends, a 16-year-old girl who was on the phone with him when Zimmerman gave chase. She has has given ABC News a terrifying account of the teenager's final moments.

He said this man was watching him, so he put his hoodie on, Martin's friend, who asked to remain anonymous, said. He said he lost the man...I asked Trayvon to run.

Suddenly, Zimmerman reappeared, cornering Martin. His friend said she was terrified.

Trayvon said, 'What, are you following me for,' and the man said, 'What are you doing here,' his friend recalled. Next thing I hear is somebody pushing, and somebody pushed Trayvon because the head set just fell. I called him again and he didn't answer the phone.

The line went dead.

ABC News has since obtained Martin's phone logs, which show that he had indeed been talking to the girl when he was followed. The conversation occurred some five minutes before police arrived on the scene to find the teenager shot in the chest.

Martin's mother and father told ABC that they could only imagine how frightened their son must have been when a large man began following him in his car, then got out to confront him. What makes it worse, they assert, is the knowledge that he almost got away.

He knew he was being followed and tried to get away from the guy, and the guy still caught up with him, his father said. And that's the most disturbing part. He thought he had got away from the guy and the guy back-tracked for him.

5. 'Stand Your Ground': Why Zimmerman Hasn't Been Arrested

That George Zimmerman remains a free man after shooting an unarmed teen is one that has baffled and outraged national readers since the story broke last week.

The case is shrouded in questions. Why did Zimmerman pursue Martin, especially after a 911 operator told him to stay in his car and wait for a police officer? Why did he take his gun with him when he got out of his SUV? Who initiated the struggle, and who called for help?

But one question, in some ways, rises above all the others. Even if Zimmerman felt that his life was threatened, how can it ever be a case of self-defense when the shooter is the one pursuing the alleged threat?

The basis for defending Zimmerman's actions, and what keeps him from being held by local police, is Florida's state self-defense law, known as Stand Your Ground.

It grants immunity to citizens who act to protect themselves, if they have reasonable fear that they may be seriously hurt or killed; and the statute's standards for what constitutes reasonable fear are notoriously lax.

Chief Lee told the New York Times this week that he'd welcome a federal investigation into the shooting death of Trayvon Martin, but claimed his hands were tied by the stand your ground statutue.

The evidence doesn't establish so far that Mr. Zimmerman did not act in self-defense, Lee said. We don't have anything to dispute his claim of self-defense at this point.

The effect this law may have had on Zimmerman, however, a licensed gun-owner studying criminal law, may be imagined. As a self-appointed neighborhood watch captain, the 28-year-old violated many of the major principles of the Neighborhood Watch manual when he followed Martin.

It should be emphasized to members that they do not possess police powers, and they shall not carry weapons or pursue vehicles, the manual states. They should also be cautioned to alert police or deputies when encountering strange activity. Members should never confront suspicious persons who could be armed and dangerous.

Brian Tannebaum, a Florida criminal defense lawyer who has been following the case, said he's worried Zimmerman is only the beginning.

Stand Your Ground is a law that has really created a Wild West type environment in Florida, Tannebaum told the Times. 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.

Meanwhile, state Sen. Oscar Braynon, has called for hearings or a select committee to take another look at the stand your ground rule and clarify what constitutes self-defense under the law, which was enacted in 2005.

6. Celebrity Activism Spurs National Media Attention... Except For Fox News?

The story of the Florida shooting of Trayvon Martin by neighborhood watch captain George Zimmerman only really caught national media attention last week, after Martin's parents had spent two weeks fighting to have Zimmerman arrested for murder.

More than 435,000 people, many alerted by social media including tweets sent out by celebrities including Spike Lee and Wyclef Jean, have signed a petition on social action website Change.org calling for Zimmerman's arrest. The campaign to draw attention to the case is the third largest petition in Change.org's history, a spokeswoman for the site said.

The Rev. Al Sharpton was scheduled to attend a town-hall-style meeting in Sanford on Tuesday evening.

The family's lawyer, Crump, said public pressure has prompted some Florida legislators to consider a change in the stand your ground legislation.

People all over the world, more than 400,000 people, said we demand you make an arrest. That's what is building pressure to look at it, Crump told the Christian Science Monitor.

It's also what has prompted every major news source in the nation to focus in on the story -- all except one.

According to News One and Think Progress, Fox News has virtually ignored the Florida shooting story since it first appeared in the news on Feb. 27, the day after Martin was shot.

A comparison of coverage alongside CNN and MSNBC shows by Think Progress asserts that while television shows on CNN and MSNBC have done 41 and 13 stories respectively on Martin's death up to March 19, Fox News has done only one story on the case. There have been no reports as to whether Fox News radio shows the same apparent pattern.

7. The Justice Department has gotten involved.

For weeks, Trayvon Martin's family has been calling on the U.S. Department of Justice to get involved in the Florida shooting. As of this week, the DOJ is finally on the case.

In a statement released Monday night, the Justice Department confirmed that its Civil Rights Division, in conjunction with the Federal Bureau of Investigation (FBI), would investigate Martin's death.

The department will conduct a thorough and independent review of all of the evidence and take appropriate action at the conclusion of the investigation, Xochitl Hinojosa, a Justice Department spokeswoman, said in a statement. The department also is providing assistance to and cooperating with the state officials in their investigation into the incident.

The Justice Department will also send members of its Community Relations Service to Sanford to try to defuse escalating tensions within the community.

The case had already been turned over to the state attorney's office last week after Sanford police asserted that they couldn't take the case any further.

That is the circumstance we are dealing with: If we arrest, we open ourselves to a lawsuit, Sgt. Dave Morgenstern, a spokesman with the Sanford Police Department, said previously.

He, like Chief Lee, flatly denies that there has been any prejudice in the investigation. I would have to say I don't think we have conducted a racially biased investigation at all.

Morgenstern said Zimmerman was in contact with the police and has been cooperating fully.

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