Who is George Zimmerman, and why did he shoot Trayvon Martin?

George Zimmerman, the 28-year-old former altar boy whose shooting of Trayvon Martin, an unarmed black teenager, has caused a national uproar, has withdrawn into seclusion. His lawyer says he’s afraid for his life amid numerous death threats.

As Florida and federal investigators dig into the Feb. 26 shooting and the Sanford Police Department's decision to not charge Zimmerman with a crime, attempts to understand the shooting have focused on the extent to which Zimmerman's past experiences colored his decisions in the moments before Trayvon died.

Critics have called Zimmerman a racist vigilante who initiated a conflict with Trayvon largely because of a profile: A black man wearing a hoodie walking through a gated neighborhood. If the FBI finds evidence of this, Zimmerman could be charged with a federal hate crime.

A separate state investigation is slated to probe whether the state's Stand Your Ground law, the justification for not charging Zimmerman, was applied properly amid new evidence that suggests Zimmerman followed Trayvon and initiated an altercation where Trayvon got the upper hand.

Zimmerman's comments on a 911 tape from the night of the shooting that “These [expletive] always get away,” and speculation about whether he uttered a racial slur before following Trayvon on foot support the idea that Zimmerman's frustration with a crime wave in the Retreat at Twin Lakes had boiled over into vigilantism.

Zimmerman has contended that he was on his way back to his SUV after following Trayvon when he was attacked. Police found Zimmerman with a bloody nose and blood on the back of his head. His lawyer says Zimmerman's nose was broken.

At the very least, a series of 46 emergency calls made by Zimmerman over the past six years document a man vigilant about keeping his neighborhood safe and orderly. The calls include complaints about unruly people at the pool, potholes, dumped trash, and kids playing in the street. In recent months, as the neighborhood saw an uptick in crime, including burglaries and a shooting, Zimmerman's calls had focused on specific suspects, the majority of them young black men.

“The newly released police calls paint Zimmerman as a man obsessed with law and order, with the minutiae of suburban life, and with black males,” writes Mother Jones' Adam Weinstein.

But Zimmerman's family, his neighbors and his lawyer paint a different picture: That of a devoted neighbor, keen enough to protect the neighborhood that residents, in establishing a local Neighborhood Watch group last year, appointed him the captain. The organization was not registered with the national Neighborhood Watch program, but was set up with the assistance of the Sanford Police Department. Zimmerman initiated the program, according to Wendy Dorival, the department's volunteer coordinator.

Frank Taaffe, a neighbor, told CNN that Zimmerman "had a passion for the safety of our neighborhood and he demonstrated to the rest of us that one person could make a difference. And he was an average guy, just like me." In a separate interview, Mr. Taaffe told the Washington Post, “George is a good dude. He cares about this community. He’s not a vigilante out looking for trouble."

Zimmerman grew up in Manassas, Va., the son of a white father and a Latina mother. He attended a Catholic church, where he was an altar boy. In high school, he was the victim of an assault. He moved to Florida a decade ago, writing in his high school yearbook, “I'm going to Florida to work with my godfather who just bought a $1 million business.”

In 2005, when he was 21, he was charged with assault on a police officer during an altercation over the arrest of one of his friends for underage drinking in a bar, but he chose a common route for first-time offenders: A pre-trial diversion that allowed him to escape a felony conviction. Such a conviction could have precluded him from having the permit that allowed him to carry the 9 mm gun used to shoot Trayvon.

"Mr. Zimmerman was not acting outside the legal boundaries of Florida Statute by carrying his weapon when this incident occurred," Sanford Police Chief Bill Lee said recently. "He was in fact on a personal errand in his vehicle when he observed Mr. Martin in the community and called the Sanford Police Department."

The chief has also said Zimmerman had no legal duty to heed a dispatcher's warning to stay put instead of following Trayvon on foot. (Chief Lee stepped down “temporarily” this week after receiving a no-confidence vote from the Sanford City Council over his handling of the case.)

Also in 2005, his then-fiancée accused Zimmerman of domestic violence when she filed a court injunction against him. He responded with an injunction of his own. Their court battled ended when the injunctions expired in 2006.

Having worked in the past as a car salesman, Zimmerman became interested in becoming a police officer. In 2008, he attended a four-month course at the local sheriff's department.

In his application for the course, Zimmerman wrote: "I hold law enforcement officers in the highest regard and I hope to one day become one."

More recently, he was taking law enforcement courses at Seminole State College, where he had earlier studied to become an insurance agent. After the shooting, the college said it had “taken the unusual but necessary step ... to withdraw” Zimmerman from the campus, citing concerns about safety for Zimmerman and other students.

The fact that Zimmerman was known to local police has deepened suspicion that prosecutors overlooked key facts in determining that Zimmerman had a legitimate claim of self-defense under Florida's landmark Stand Your Ground law, which allows the use of deadly force and obviates any need by a citizen to retreat if they reasonably fear for their life.

"The reality is that this chief had probable cause to lock up a man who shot a boy in cold blood – because he shot a boy in cold blood – and he failed to do that," NAACP President Ben Jealous said this week.

But it's the specter of racial injustice and suspicions about Zimmerman's views on race that have sparked dozens of protests around the country and drawn international attention to Trayvon's case.

Zimmerman's father, a retired magistrate judge, fought back against those allegations in a recent letter to the Orlando Sentinel.

“The portrayal of George Zimmerman in the media, as well as the series of events that led to the tragic shooting, are false and extremely misleading," Robert Zimmerman wrote. "Unfortunately, some individuals and organizations have used this tragedy to further their own causes and agendas. George is a Spanish-speaking minority with many black family members and friends. He would be the last to discriminate for any reason whatsoever."

In an interview Friday, Zimmerman's lawyer, Craig Sonner, told CNN that Zimmerman had recently mentored a black boy, taking him out to play basketball and participating in fund-raisers at the boy's church.

“This is a case about self-defense, that's what the trial is going to be about,” Mr. Sonner said. “It's not about being angry over a racial issue.”