George Zimmerman, 28, was jailed in Sanford - the site of the shooting Feb. 26 that set off a nationwide debate over racial profiling and self-defense - on a charge that carries a minimum of 25 years in prison and a maximum sentence that could put him in prison for life.
In announcing the arrest, prosecutor Angela Corey would not discuss how she reconciled the conflicting accounts of what happened or explain how she arrived at the charges, saying too much information had been made public already. But she made it clear she was not influenced by the uproar over the past six weeks.
"We do not prosecute by public pressure or by petition. We prosecute based on the facts on any given case as well as the laws of the state of Florida," Corey said.
Martin's parents, who were in Washington when the announcement came, expressed relief over the decision to prosecute the killer of their 17-year-old son.
"The question I would really like to ask him is, if he could look into Trayvon's eyes and see how innocent he was, would he have then pulled the trigger? Or would he have just let him go on home?" said his father, Tracy Martin.
Many legal experts had expected the prosecutor to opt for the lesser charge of manslaughter, which usually carries 15 years behind bars and covers reckless or negligent killings, rather than second-degree murder, which involves a killing that results from a "depraved" disregard for human life.
The most severe homicide charge, first-degree murder, is subject to the death penalty in Florida and requires premeditation - something that all sides agreed was not present in this case.
"I predicted manslaughter, so I'm a little surprised," said Michael Seigel, a former federal prosecutor who now teaches law at the University of Florida. "But she has more facts than I do."
Zimmerman's new attorney, Mark O'Mara, said Zimmerman will plead not guilty and will invoke Florida's powerful "stand your ground" law, which gives people wide leeway to use deadly force without having to retreat in the face of danger.
The lawyer asked that people not jump to conclusions about his client's guilt and said he is "hoping that the community will calm down" now that charges have been filed.
"I'm expecting a lot of work and hopefully justice in the end," O'Mara said.
Zimmerman, whose father is white and whose mother Hispanic, turned himself in earlier in the day and will make a court appearance as early as Thursday, when his lawyer plans to ask for bail.
Corey's decision followed an extraordinary 45-day campaign by Martin's parents to have Zimmerman arrested despite his claim that he shot in self-defense. They were joined by civil rights activists such as Jesse Jackson and Al Sharpton, as well as many politicians and supporters in Sanford and cities across the nation.
Protesters wore hooded sweatshirts like the one Martin had on. And the debate reached all the way to the White House, where President Barack Obama observed last month: "If I had a son, he'd look like Trayvon."
The confrontation took place in a gated community where Martin was staying with his father and his father's fianc?e. Martin was walking back in the rain from a convenience store when Zimmerman spotted him and called 911. He followed Martin despite being told not to by a police dispatcher, and the two got into a struggle.
Zimmerman told police Martin punched him in the nose, knocking him down, and then began banging Zimmerman's head on the sidewalk. Zimmerman claimed he shot Martin in fear for his life.
A judge could dismiss the charge based on "stand your ground," legal experts said. But not if prosecutors can show Zimmerman was to blame.
"If you're the aggressor, you're not protected by this law," said Carey Haughwout, public defender in Palm Beach County.
Zimmerman's brother Robert Zimmerman told CNN on Wednesday night: "Our brother literally had to save his life by taking a life. And that's a situation nobody wants to be in, ever."
On Tuesday, Zimmerman's former lawyers portrayed him as erratic and in precarious mental condition. O'Mara, who signed on after Zimmerman's previous attorneys withdrew, said that Zimmerman seemed to be in a good state of mind but that the pressure had weighed mightily on him.
"He is troubled by everything that has happened. I cannot imagine living in George Zimmerman's shoes for the past number of weeks. Because he has been at the focus of a lot of anger, and maybe confusion and maybe some hatred, and that has to be difficult," the attorney said.
O'Mara also said the difficult case is compounded by the heavy media attention, which might make it hard to seat an impartial jury. Corey, similarly, complained: "So much information got released on this case that never should have been released. We have to protect this prosecution and this investigation for Trayvon, for George Zimmerman."
Corey, the prosecutor in Jacksonville, was appointed to handle the case by Republican Gov. Rick Scott after the local prosecutor disqualified himself. She has tried hundreds of homicide cases and is known for tough tactics aimed at locking up criminals for a long time and making it difficult to negotiate light plea bargains.
The U.S. Justice Department's civil rights division is conducting its own investigation. But federal authorities typically wait until a state prosecution is complete before deciding how to proceed.
Tensions had risen in recent days in Sanford, a town of 50,000 outside Orlando. Someone shot up an unoccupied police car Tuesday as it sat outside the neighborhood where Martin was killed. But as the hour of the prosecutor's announcement neared, the Martin family and their lawyer pleaded for calm.
Outside Sanford City Hall, Stacy Davis, a black woman, said she was glad to see Zimmerman under arrest.
"It's not a black or white thing for me. It's a right or wrong thing. He needed to be arrested," she said. "I'm happy because maybe that boy can get some rest."
Farrington reported from Tallahassee, Fla. Associated Press writers Curt Anderson in Miami, Kyle Hightower in Sanford and Mike Schneider in Orlando contributed.