Here's what we know:
Zimmerman told the police he was in his truck, stepped out of it to check the name of the street he was on, and then Tayvon Martin jumped him from behind. Zimmerman had to defend himself, so he shot the 17 year old.
The 911 tapes tell a different story: Zimmerman saw Martin, saw Martin begin to run from him, and he got out of his truck in pursuit. When he caught up with Martin, he shot him. "What happened" is clear from what Zimmerman told police.
A friend of Martin's was on the phone with him as he noticed Zimmerman, and as he walked quickly (he told his friend he wouldn't run) away from Zimmerman, who pursued him. She heard Martin ask "Why are you following me?" and heard Zimmerman (presumably) say: "What are you doing around here?" Almost immediately after that, Martin was shot and killed.
The police didn't bother to check Martin's cell phone for recent calls, or to notice he was on the phone as he was shot (making the person on the other end of that call a potential witness). They took Zimmerman's story at face value even though it contradicts the 911 tapes.
To sum up: for Zimmerman's story to be true, he had to get out of the truck to tell the 911 operator the name of the street he was on, get back in his truck, get out again to pursue Martin, who somehow at that point vanished from running down the street away from Zimmerman, and appeared behind him, threatening Zimmerman so that he had to shoot to defend himself.
The Dept. of Justice has noted it must have evidence of intent in order to pursue federal charges; that negligence (such as the gun going off in a struggle, without Zimmerman intending to fire) cannot be charged under federal law. But Zimmerman's statement to the police is that he intended to fire. If Zimmerman changes his story now in order to evade federal prosecution (such as saying the shooting was accidental), he loses all credibility (not that he should have any, in light of the evidence that contradicts his statement to the police).
And the authors of the so-called "stand your ground law" argue it doesn't protect Zimmerman from a situation he placed himself in. However, Jeffrey Bellin's analysis of the requirements of the law in Florida points out raising the defense puts a burden on the prosecution: to disprove the defense.
Critical to this determination will be evidence reflecting: how the confrontation began and how the suspect acted after the confrontation (prosecutors often look for actions such as flight or a cover-up that indicate a "consciousness of guilt"). Perhaps most critically, investigators will compare all the evidence (physical and otherwise) with the suspect's statement (if any) about what happened.Which, I think, is where the evidence known so far gets Zimmerman in trouble. His statement to police contradicts the 911 phone call he made; and the statements of Martin's girlfriend contradict Zimmerman's statement, too. The evidence of the phone calls indicate who started the confrontation, and who escalated it, and that Zimmerman decided, after he'd fired, to claim self-defense.
There is also the absolutely grotesque dereliction of duty evident in the Sanford Police Department's handling of the case. Trayvon Martin was treated as a John Doe, despite the fact he had a cell phone which could have been investigated for information (the family brought out the story of Trayvon's girlfriend, not the police). Trayvon was tested for drugs and his criminal background checked, not Zimmerman's.
And in all of this, the family has called consistently for one thing: the arrest of George Zimmerman. Not his conviction, or even his execution: simply his arrest. Which says a lot about the nature of the Martin family, but also a lot about race in America. The City Manager of Sanford, Florida was on Lawrence O'Donnell's show last night, and while he admitted the Police Chief of Sanford serves at the pleasure of the City Manager, he refused to consider firing Bill Lee without a full investigation: in other words, not until sometime in the far future. Which sounds reasonable, in one sense; in another, it indicates the politics of this town in Florida. Even City Managers have to take politics into account, and right now George Zimmerman is seen as white (he claims Hispanic), and Trayvon Martin is a dead black boy in a hoodie. A white family would be demanding rough justice; a black family knows all they can demand, is justice.
They are right to ask for no more than that. But they also understand society entitles them to no more than that. A white family might well consider they are entitled to vengeance. The arguments for the "stand your ground" law in Florida were based on that sense of entitlement: you have to defend yourself in a situation of threat. The problem is, of course, what constitutes a "threat;" and also, who is threatened. If Trayvon Martin had also had a gun, and ended up shooting George Zimmerman, does anyone really think he'd be free right now because of "self-defense"?