Update: The Florida state attorney has sent the Trayvon Martin case to a grand jury that will be called to session on April 10, Think Progress is reporting

The federal government's Civil Rights Division and the FBI will launch their own investigation into the shooting death of Trayvon Martin, the Florida teenager whose killer has still not been arrested after claiming self-defense. Martin's family had been pushing for three weeks for the Justice Department to get involved and the increasing national attention to the story has led to mounting pressure from the community, the media, and celebrities that has forced the Attorney General to respond.

For those still catching up, Martin was shot and killed last month by George Zimmerman, a neighborhood watch captain in the Orlando suburb of Sanford. Zimmerman (who is Latino) spotted Martin (who is black) walking down the street by himself on the night of February 26. (Martin did not live in the gated community, but was there visiting his father at his father's girlfriend's house.) What happened next is in dispute, but at some point a confrontation occurred and Zimmerman, who is licensed to carry a concealed gun, shot and killed Martin. Even though Martin was unarmed and nearly 80 pounds lighter, Zimmerman claimed self-defense under Florida's liberal "stand your ground" law. After reviewing the case, the Sanford police accepted Zimmerman's version of events and declined to press charges.

The case became national news over the weekend after a story in The New York Times highlighted many of the problems with Zimmerman's story and the investigation done by the Sanford police department. A review of a 911 call made by Zimmerman to police after spotting Martin, shows that Martin ran away from Zimmerman after being spotted and that Zimmerman pursued him, despite being told by the police dispatcher not to follow. On another 911 call made by a neighbors who witnessed the incident, someone can be heard pleading for help before a gun shot rings out. Zimmerman claimed he was the one who cried out, but that is in dispute.

There are also problems with the police's initial characterization of  Zimmerman as "squeaky clean," which was part of the justification for not arresting him. He had a previous arrest for assaulting a police officer and is not a member of registered neighborhood watch group. He was also not tested for drugs or alcohol, which is a common procedure following any shooting death. Sanford's police have also been accused in the past of mishandling or covering up other assaults on minorities. Some are also claiming that Zimmerman may have used a racial slur to refer to Martin during the 911 call, though the audio doesn't make that clear.

Mother Jones has a pretty thorough rundown of the entire case, including audio of the 911 calls.

Despite the intervention of the Feds, the case remains a difficult one to pursue, because of Florida's law regarding self-defense. Under controversial rules passed in 2005, state residents are allowed to use deadly force against anyone they believe to be a threat, while also being under no obligation to retreat or back down before resorting to violence (as self-defense laws in most state require.) Also, under these rules a shooter does not have to prove that he or she was actually under threat, merely that they believed they were. It's burden is on the state to prove that they were in no danger before acting.

That's just one reason why, even as the Justice Department prepares to step in with an independent investigation, they already appearing to tamping down expectations that Zimmerman will face a trial and conviction. The formal statement goes to some length to explain the burden prosecutors will face:

With all federal civil rights crimes, the government must prove beyond a reasonable doubt that a person acted intentionally and with the specific intent to do something which the law forbids – the highest level of intent in criminal law.  Negligence, recklessness, mistakes and accidents are not prosecutable under the federal criminal civil rights laws.  

In other words, it won't be easy to prove that Zimmerman broke the law, since he's the only one who knows what happened. The only thing that is clear, is that the case is not going away quietly.